MacBook Pro lags when at 0% and charging
This is known/expected behavior as the processor lags noticeably when the battery is removed on any recent MacBook to MacBook Pro. If you can avoid getting the battery below 10% or replace the battery - all normal performance should resume.


Is it possible to disable CPU throttling on a MacBook Pro with a completely dead (not recognized) battery?
How to disable the SpeedStep when using MacBook Pro without a battery?