Hi David,
Me and @cristi have done some investigation, and we can reproduce some pretty major issues with Sending from Unity. 
We have had some limited success disabling the attaching of SpeckleProperties since this was leading to super large payloads, but this didn’t totally solve the issue.
We are a little stumped, as this is quite hard to debug, but I’m going to try and create some bare-bones tests that should help isolate the problem.
I’ll keep you informed as to our progress, but it does seem that the current main branch is affected.
Thanks for bringing this to our attention, and we hope to solve this shortly.