Joe Obbish has another post looking at sub-optimal columnstore index performance:
It is possible to see a scalability bottleneck in the form of high average wait time for the
RESERVED_MEMORY_ALLOCATION_EXTwait if a highly concurrent workload is run on a server that consumes memory with batch mode operators. I believe that the severity of the bottleneck depends on unknown factors in the server’s initial memory state and the rate of memory actually used by queries to run batch mode operations. This blog post shares a reproduction of the issue along with a call to action.
If you use clustered columnstore indexes, check out Joe’s User Voice entry.