Write a topic on tuning Scrapy performance

[Gallaecio]

Cover how to identify a bottleneck (network input/output, CPU, disk I/O, memory) and how to improve Scrapy’s performance for any given bottleneck.

Cover PYTHONOPTIMIZE where relevant, but:

• Explain that PYTHONOPTIMIZE=2 (-OO) cannot be used with Scrapy, as scrapy indirectly depends on pycparser (scrapy → cryptography → cffi → pycparser), which does not support this optimization level (their README explains why).
• When covering PYTHONOPTIMIZE=1 (-O), be realistic about potential gains.

[Gallaecio]

Related: https://github.com/scrapy/scrapy/pull/5566