The History feature is not only useful but absolutely essential.
But you need to know, however, if you are working on large (Many Mbs) pictures and your resources are low it will slow down because it will be completely redrawing the picture for every change you make.
So if you are working on a 25meg pic, everytime you change even one pixel that is another 25meg being used up by the memory/swap file.
You can adjust this setting in 'Preferences' but really I find it useful to have it as large as possible.
Getting confused now...History feature ? 25Mb picture..seems very large. The majority of my pics are far less than that even at High Res ! Unused memory 41% ? Does this mean hard disk space ? The Ram question has still not been addressed although both myself and wee eddie have asked.