Crazy! I don’t use 2015 as my primary version yet, but I’ll do some tests. Haven’t had anything like that in 2013/2014. Must be a memory leak when using a custom artisan tool. Sounds like, with your edit, that it must be a garbage collection issue. Like, the undo stack is getting ridiculously large until a garbage collection happens. I’ll check to see if I can do a force flush or limit what the undo stack saves for that paint weights tool.