Thanks for investigating!
I have been pasting a lot of cover art art (from clipboard) since I reported the stalling issue (with a huge log file) to you via email and I have not seen any more issues with stalling although I have pasted several equally huge jpg files. MC has also behaved recently and kept up a reasonable speed when pasting. I have not checked memory usage during the pasting sessions, but after a massive session MC uses less than 100 MB RAM. Very good.
The perceived decreased in pasting speed could have been due to this issue I just recently discovered. I have a split view with two "view sets" ("rows") each with several tabs (one of the two view sets is locked). When doing cover art tasks I completely hide the locked view set. Before I hide it I select a tab and a pane search list item in that tab/view where there is no files to view (my new music to tag tab/search list), because every tag/library changing operation in the other view set is slow if a tab based on complex expressions and many files to view is active. Now I discovered that the hidden view set sometimes loses the selection of the pane search list item and reverts to show all files (i.e. pane selection lost), which slows down tag/library changes in the visible view set. I do not recall having seen the not used view set lose focus when it was visible. I have not been able to reproduce the loss of focus issue, but I think a switch of tabs, or perhaps a change of sort order, in the visible view set triggers it.
In short, no more stalling to report despite intense provocation and the tardiness of pasting might have been due to something happening in a hidden view set (which is a completely different issue than the heading of this thread). I report back if I find a way to reproduce.