I honestly do think you're lagging, but there are most likely multiple factors making the lag occur; your computer + your WiFi may not be giving you great ping and map editor may just be overloading it a little. You can clear your cache and find ways to speed your computer up, and it may help.
Otherwise, it really may be a glitch in the standalone; I'm sure if that's the case a dev will see this and fix it! If it doesn't happen in other standalone versions or in browser, then yes, this is the case.
In the meantime, please use browser or another version of standalone until this is fixed, or try the advice in the first paragraph and see if it helps, but there isn't much that can be done in the meantime aside from that.
(I hope you understand my English well.)
All solutions offered;
thread closed.