Backport #6185: optimize mempool

[acud]

Motivation

Improves some aspects of the mempool handling.

Description

Test Plan

TODO

• Explain motivation or link existing issue(s)
• Test changes and document test plan
• Update documentation as needed
• Update changelog as needed

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.5%. Comparing base (e64a1dc) to head (9228700).
Report is 4 commits behind head on v1.6.

Additional details and impacted files

@@           Coverage Diff           @@
##            v1.6   #6186     +/-   ##
=======================================
- Coverage   81.6%   81.5%   -0.1%     
=======================================
  Files        301     301             
  Lines      32323   32342     +19     
=======================================
  Hits       26389   26389             
- Misses      4214    4230     +16     
- Partials    1720    1723      +3     

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