I’ve noticed some unexplainable weirdness with Dynamo and performance
Refer Dynamo: performance
In my case, there seemed to be a hard ceiling/threshold somewhere between 20,000 and 30,000 objects
A little different to your problem of nulls.
I ended up just working around the problem.
Could you split it into smaller chunks e.g move rooms into separate worksets and unload the worksets/run Dynamo ?
Andrew