It replaces game code with almost identical code (Harmony Patches), but with more performance measurements

Those measurements are captured by Capture Window, which is basically a queue of slices of measurements, each slice contains measurements from 1 frame, update frame or draw frame

Then those measurements are sorted, transformed from ticks to ms and displayed in GUI

Those Harmony Patches can either run code version with measurements or vanilla code, it depends on if corresponding Capture State is active

You can see and toggle those Capture States on GUI map

If Capture State is inactive or GUI window is closed then mod will run vanilla code which should have no impact on performance

If you have other mods that replaces game code (e.g Remove All) then Showperf Extensions will take presidence

Also you can set "By ID" for each Capture State and limit capture to some specific submarine type
Each Capture State have "By ID", but not all Harmony Patches have code to react to them, only the ones where i wasn't too lazy to make them

Capture Window by default captures 100 frames and currently have 3 modes:

• "mean" - rotates the queue of slices, adds first and substracts last, then gives total / number of slices (Frames)
  
• "sum" - just continuously adds first slice to total
  
• "spike" - rotates the queue, adds values from first slice to total if it's bigger and removes it when it leaves the queue
  If it has only 1 frame then it's just replaces stored values with max forever

Amount of Window frames shouldn't have impact on performance, only on the memory usage