[PR #6274/254d0e7d backport][3.9] Clarify tiny timeouts behavior and how it can affect performance

[patchback]

This is a backport of PR #6274 as merged into master (254d0e7).

See also #6008

[codecov]

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@@          Coverage Diff           @@
##             3.9    #6304   +/-   ##
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  Coverage       ?   93.30%           
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  Files          ?      103           
  Lines          ?    30362           
  Branches       ?     2729           
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  Hits           ?    28328           
  Misses         ?     1857           
  Partials       ?      177           

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unit	93.22% <0.00%> (?)

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