not sure this is known already, the upload of the parameters with MP can become extremely slow.
I think I've read in the issues or PR, and also in the mai-01-Dev report about a parameter upload problem, but these reports I understood to mean upload in flight. Also, it read to me as if the problem there was solved. So, to be sure, I've flashed the latest AP3.6-dev v2 of tonight, as well as the latest MP (not beta, official).
I've made a short video, since I considered this faster than trying to explain that here: https://www.youtube.com/watch?v=tqslbQcylE0
I had that problem also with the previous 3.6-dev I had flashed (ca. 2-3 weeks old), but not with previous versions. The behavior is reproducible.
I'd like to mention that with UAVCAN enabled and with uc4h UAVCAN nodes connected, I can get all sorts of instable behavior, the ardupilot node having sometimes problems to appear, suddenly appear&reappearings, the 2nd set of tones to very late, extreme slow downs and complete hang-ups of MP at parameter upload, and in one case even a crash of MP upon trying a connect ... however, I could not see the systematics so far, nor is it reproducible but just happens more often or not, all this looks connected to me, but that's not actually clear. So, I just mention this set of issues here, but currently feel unable to provide more helpful info.