Have you tried profiling it yet? I think it would be helpful to see the graphs from the first A->B transition and the graphs from the second A->B transition to see if it's a CPU problem or a GC (garbage collector) problem.
If you're getting your RAM readings from the profiler, then disregard the following, otherwise if you're it from Task Manager it's useful to remember that Task Manager gives a pretty rough estimate of the current RAM usage, and isn't really suitable for profiling memory usage.
Yeah, I'll give the Profiler a shot. It's just that within the Editor, the entire Async loading coding is finicky as well.
Edit: Hrm, within the Editor the RAM usage is a bit higher too, under the vague "Unity" name. For the rest, the Profiler isn't all that great since Async loading does not work within the Eidtor itself and therefore there's nothing Async about it. I can't see what happens while it's loading, the entire thing freezes.