Hi Dan,
I got a chance to review the traces together with some devs. They confirmed to me that oftentimes these hitches happen due to the design of the Windows allocator and how UE is currently handling resource/heap allocations on the critical path. As you pointed out, the affected allocations enter a critical section and take a lock, causing a stall for other resources. What is curious, though, is that the allocations causing these hitches also seem larger than 64MB for some dynamic primitives, which is a bit concerning. Can you please let me know why you are making this large of an allocation? Fixing this will likely require a refactor of the engine code so as not to take the lock before entering the critical section, but perhaps we can find an alternate solution.