perf: reduce import time - draft

[anakin87]

Motivation

Motivated by recent issues (#8649, #8650, and #8704), I investigated the import times of Haystack.

I used some raw scripts (attached in this draft PR) and commands like
python -X importtime -c "import haystack" to analyze cumulative import times of packages.

Findings

• Our LazyImport context manager behaves like a DeferImportError context manager. If a package you import is available, it gets imported immediately. If not, the import error is caught and only raised later when calling .check(). Maybe this is something known, but it was surprising to me.
• As a result the more libraries installed, the slower the imports (since more packages get loaded).
• Our telemetry surprisingly imports heavy dependencies like torch and transformers.

An exploratory solution

I explored low-effort ways to improve this:

• remove heavy imports from telemetry
• make modules import in __init__.py of packages actually lazy
  • I only did this for the utils package, that triggered the torch import

Results

Tested on Ubuntu 22.04, Python 3.10.12, averaged over 30 runs.

I focus on User time and System time, as they capture actual CPU time, irrespective of parallelism and system load (reference).

Dependencies

• Minimal = pip install haystack-ai
• Test = All dependencies required for tests (pyproject)

Haystack	Dependencies	User Time	Sys Time	Total Time
Current (35788a2)	Minimal	2.114s	0.031s	2.145s
Current (35788a2)	Test	3.236s	0.185s	3.420s
This PR	Minimal	2.098s	0.036s	2.134s
This PR	Test	2.265s	0.050s	2.316s

Next Steps

I believe that these results can encourage discussion.

• Should we rename LazyImport or transform it into a truly lazy importer (difficult if we want to keep its API unchanged)?
• Do we want to use real lazy imports in the __init__.py of packages?

[coveralls]

Pull Request Test Coverage Report for Build 13204178773

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 6 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 92.284%

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Files with Coverage Reduction	New Missed Lines	%
utils/requests_utils.py	6	0.0%

Totals
Change from base Build 13203068476:	-0.02%
Covered Lines:	9125
Relevant Lines:	9888

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💛 - Coveralls