The Bug

Average FPS and TPS in 1.16.2 release is lower, than in 1.16.2 release candidate 1.
Worth to mention, rc2 (which became 1.16.2 release) only reverted one of the rc1 changes (in terms of anything that can affect performance)

It all causes lag spikes to appear a bit more often than usual, and makes them actually affect the game in form of reading mouse and (probably, but hard to confirm) keyboard output twice, which leads, but isn't limited to:

• dubble-read mouse output (block placing/breaking in creative; using items such as firework, filled buckets, etc.; attacking)
• fluids desync appears even more often, than since 1.15
• mobs' AI micro-lags visibly more often.

This was NOT a thing in 1.16.2 Release candidate 1.

I know lag spikes exist in 1.16.2 rc1, as seen on the screenshots below, but they didn't affect the game that harshly.

Screenshots from testing below were made on newly created world with the same world seed and world type (waited around 15 seconds on both, before started making screenshots)