When there are multiple conversation/chat tabs open, sometimes switching between tabs causes Spark to freeze up for maybe 10 - 15 seconds. I have multiple tabs open frequently, and I'd say this freeze up happens at least once a day or more. When it happens, I just wait for a bit, then it snaps out of it and resumes working again.
I've seen this issue for several Spark versions now. I just loaded 2.7.2, and the problem still occurs.