@@ -4,6 +4,9 @@ All notable changes to this project will be documented in this file.
 
 ## [Unreleased]
 
+### Features
+- New `ReSettings.EnableParallelRuleCompilation` (default `false`). When `true`, workflow rules are compiled in parallel during registration, materially reducing warmup time for workflows with many thousands of rules. Silently falls back to serial compilation when combined with `UseFastExpressionCompiler = true` (which regresses ~3× under contention) or for workflows below an internal scheduling-cost threshold. Builds on the warmup work in #740 (#741).
+
 ## [6.0.1-preview.2]
 
 ### Features
 
@@ -30,6 +30,7 @@ internal ReSettings(ReSettings reSettings)
             UseFastExpressionCompiler = reSettings.UseFastExpressionCompiler;
             EnableExceptionAsErrorMessageForRuleExpressionParsing = reSettings.EnableExceptionAsErrorMessageForRuleExpressionParsing;
             AutoExecuteActions = reSettings.AutoExecuteActions;
+            EnableParallelRuleCompilation = reSettings.EnableParallelRuleCompilation;
         }
 
 
@@ -98,6 +99,22 @@ internal ReSettings(ReSettings reSettings)
         /// run actions yourself (e.g. via ExecuteActionWorkflowAsync) for selective control. See #596.
         /// </summary>
         public bool AutoExecuteActions { get; set; } = true;
+
+        /// <summary>
+        /// When true, rules within a workflow are compiled in parallel during registration.
+        /// Significantly reduces warmup time for workflows with many thousands of rules.
+        /// </summary>
+        /// <remarks>
+        /// Silently falls back to serial compilation when:
+        /// <list type="bullet">
+        /// <item><see cref="UseFastExpressionCompiler"/> is also <c>true</c> — FastExpressionCompiler
+        /// regresses ~3× under parallel contention, so the engine declines to parallelize that mix.</item>
+        /// <item>The workflow has fewer enabled rules than an internal threshold (32) where
+        /// the dispatch cost outweighs the speedup.</item>
+        /// </list>
+        /// Default: <c>false</c>.
+        /// </remarks>
+        public bool EnableParallelRuleCompilation { get; set; } = false;
     }
 
     public enum NestedRuleExecutionMode
 
@@ -35,6 +35,10 @@ public class RulesEngine : IRulesEngine
         private readonly RuleCompiler _ruleCompiler;
         private readonly ActionFactory _actionFactory;
         private const string ParamParseRegex = "(\\$\\(.*?\\))";
+
+        // Below this rule count, Parallel.For's scheduling cost exceeds the speedup from
+        // distributing CompileRule across threads. See ReSettings.EnableParallelRuleCompilation.
+        private const int MinRulesForParallelCompilation = 32;
         #endregion
 
         #region Constructor
@@ -399,9 +403,43 @@ private bool RegisterRule(string workflowName, params RuleParameter[] ruleParams
                     _rulesCache.AddOrUpdateGlobalParamsDelegate(compileRulesKey, globalParamsDelegate);
                 }
 
-                foreach (var rule in workflow.Rules.Where(c => c.Enabled))
+                var enabledRules = workflow.Rules.Where(c => c.Enabled).ToArray();
+                var compiledFuncs = new RuleFunc<RuleResultTree>[enabledRules.Length];
+
+                // Parallel compilation helps only when:
+                //   - the user opted in,
+                //   - they're not also on UseFastExpressionCompiler (which regresses ~3× under
+                //     parallel contention; FEC's internal locking serializes effort), and
+                //   - there are enough rules to amortize Parallel.For's scheduling cost.
+                var shouldParallelize = _reSettings.EnableParallelRuleCompilation
+                                     && !_reSettings.UseFastExpressionCompiler
+                                     && enabledRules.Length >= MinRulesForParallelCompilation;
+
+                if (shouldParallelize)
+                {
+                    try
+                    {
+                        Parallel.For(0, enabledRules.Length, i => {
+                            compiledFuncs[i] = CompileRule(enabledRules[i], workflow.RuleExpressionType, ruleParams, globalParamExp);
+                        });
+                    }
+                    catch (AggregateException ae) when (ae.InnerExceptions.Count > 0)
+                    {
+                        // Preserve the serial-compilation contract: the first rule that fails
+                        // to compile surfaces its own exception, not an AggregateException.
+                        System.Runtime.ExceptionServices.ExceptionDispatchInfo.Capture(ae.InnerExceptions[0]).Throw();
+                    }
+                }
+                else
+                {
+                    for (var i = 0; i < enabledRules.Length; i++)
+                    {
+                        compiledFuncs[i] = CompileRule(enabledRules[i], workflow.RuleExpressionType, ruleParams, globalParamExp);
+                    }
+                }
+                for (var i = 0; i < enabledRules.Length; i++)
                 {
-                    dictFunc.Add(rule.RuleName, CompileRule(rule,workflow.RuleExpressionType, ruleParams, globalParamExp));
+                    dictFunc.Add(enabledRules[i].RuleName, compiledFuncs[i]);
                 }
 
                 _rulesCache.AddOrUpdateCompiledRule(compileRulesKey, dictFunc);
 
@@ -0,0 +1,117 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using RulesEngine.Models;
+using System.Diagnostics.CodeAnalysis;
+using System.Linq;
+using System.Threading.Tasks;
+using Xunit;
+
+namespace RulesEngine.UnitTest
+{
+    [ExcludeFromCodeCoverage]
+    public class ParallelRuleCompilationTest
+    {
+        // Larger than the internal MinRulesForParallelCompilation threshold so parallel mode actually engages.
+        private const int RuleCount = 64;
+
+        private static Workflow BuildLargeWorkflow() => new Workflow
+        {
+            WorkflowName = "wf",
+            Rules = Enumerable.Range(0, RuleCount)
+                .Select(i => new Rule { RuleName = $"R{i}", Expression = $"input1 >= {i}" })
+                .ToArray()
+        };
+
+        [Fact]
+        public async Task ParallelCompilation_ProducesIdenticalResultsAsSerial()
+        {
+            var workflow = BuildLargeWorkflow();
+
+            var serialEngine = new RulesEngine(new[] { workflow },
+                new ReSettings { EnableParallelRuleCompilation = false });
+            var parallelEngine = new RulesEngine(new[] { workflow },
+                new ReSettings { EnableParallelRuleCompilation = true });
+
+            var serial = await serialEngine.ExecuteAllRulesAsync(
+                "wf", new[] { RuleParameter.Create("input1", 32) });
+            var parallel = await parallelEngine.ExecuteAllRulesAsync(
+                "wf", new[] { RuleParameter.Create("input1", 32) });
+
+            Assert.Equal(serial.Count, parallel.Count);
+            for (var i = 0; i < serial.Count; i++)
+            {
+                Assert.Equal(serial[i].Rule.RuleName, parallel[i].Rule.RuleName);
+                Assert.Equal(serial[i].IsSuccess, parallel[i].IsSuccess);
+            }
+        }
+
+        [Fact]
+        public async Task ParallelCompilation_PreservesExceptionMessage_WhenRuleExpressionThrows()
+        {
+            // Inject a deliberately broken rule into a large-enough workflow that the parallel
+            // path engages, then assert the per-rule ExceptionMessage explains the underlying
+            // failure rather than leaking an AggregateException.
+            var rules = Enumerable.Range(0, RuleCount)
+                .Select(i => new Rule { RuleName = $"R{i}", Expression = "input1 >= 0" })
+                .ToList();
+            rules[5].Expression = "input1.NoSuchMember.Foo()";
+
+            var workflow = new Workflow { WorkflowName = "wf", Rules = rules };
+            var engine = new RulesEngine(new[] { workflow },
+                new ReSettings { EnableParallelRuleCompilation = true });
+
+            var results = await engine.ExecuteAllRulesAsync(
+                "wf", new[] { RuleParameter.Create("input1", 1) });
+
+            var broken = results.Single(r => r.Rule.RuleName == "R5");
+            Assert.False(broken.IsSuccess);
+            Assert.False(string.IsNullOrEmpty(broken.ExceptionMessage));
+            Assert.DoesNotContain("AggregateException", broken.ExceptionMessage);
+        }
+
+        [Fact]
+        public async Task ParallelCompilation_FallsBackToSerial_WhenFastExpressionCompilerEnabled()
+        {
+            // The flag combination is permitted at construction time (back-compat), but the
+            // engine declines to parallelize and silently uses the serial path. We can only
+            // assert correctness here (results match serial). The fallback itself is
+            // observable in benchmarks, not in functional tests.
+            var engine = new RulesEngine(new[] { BuildLargeWorkflow() },
+                new ReSettings
+                {
+                    EnableParallelRuleCompilation = true,
+                    UseFastExpressionCompiler = true
+                });
+
+            var results = await engine.ExecuteAllRulesAsync(
+                "wf", new[] { RuleParameter.Create("input1", 100) });
+
+            Assert.Equal(RuleCount, results.Count);
+            Assert.All(results, r => Assert.True(r.IsSuccess));
+        }
+
+        [Fact]
+        public async Task ParallelCompilation_FallsBackToSerial_ForSmallWorkflowsBelowThreshold()
+        {
+            // Below the minimum threshold, the engine declines to parallelize. Verify a
+            // 5-rule workflow still works correctly with the flag enabled.
+            var workflow = new Workflow
+            {
+                WorkflowName = "wf",
+                Rules = Enumerable.Range(0, 5)
+                    .Select(i => new Rule { RuleName = $"R{i}", Expression = $"input1 >= {i}" })
+                    .ToArray()
+            };
+            var engine = new RulesEngine(new[] { workflow },
+                new ReSettings { EnableParallelRuleCompilation = true });
+
+            var results = await engine.ExecuteAllRulesAsync(
+                "wf", new[] { RuleParameter.Create("input1", 3) });
+
+            Assert.Equal(5, results.Count);
+            // input1=3 means R0..R3 succeed, R4 fails. Same outcome whether serial or parallel.
+            Assert.Equal(4, results.Count(r => r.IsSuccess));
+        }
+    }
+}