Persisting latency between arduino and pyserial on windows 10 pro

Projects Interfacing w/ Software on the Computer
2

A number of Arduinos are reset when you open the serial port which in turn activates the bootloader. On those Arduinos, the bootloader waits for an upload or times out after a few seconds.

You don't mention which Arduino you're using; so the above might not be relevant. And you don't show your Arduino code.

Some ideas are in these two related topics by Robin2

  1. Serial Input Basics - updated
  2. Demo of PC-Arduino comms using Python

Note:
I'm not a Python programmer