I have noticed that Clipboard History has a performance problem. I normally trigger the Clipboard History with a hot key and initially the response is immediate. However over time the response gets worse. Earlier today it took about 20 seconds for the window to show.
When I check the history window I can see that is has the maximum number of entries being 200.
I did remove all the named clipboards and that did not help to solve the performance problem.
My solution now, is that I completely kill Keyboard Maestro (including the engine) after which the history is reduced to 1 entry. At that moment the response is instant again.
Given that 200 entries is the default maximum, I would not expect that this is an issue and that Keyboard Maestro would not be able to handle that many entries.
By removing all the named clipboards, I was trying to remove as much influence as possible. What I am copying will vary from day to day. Sometimes this is graphics, sometimes this is code and sometimes it will be a mix.
As you suggest, next time it is sluggish I will inspect and look for large graphics and delete them individually to see if that is the root cause or not.