Atomic updates too slow in Solr 8 vs Solr 7

Started experiencing slowness with updates in production after upgrading from Solr 7.7.2 to 8.11.1. Upon comparing the performance it turns out that indexing 20 million docs via atomic updates through the same client program (running 15 parallel threads indexing in batches of 1000) takes below time:
 
Solr 7 : 78 mins
Solr 8:  370 mins 
 
Environment details:

• Java 11 on Windows server
• Xms1536m Xmx3072m
• Indexing client code running 15 parallel threads indexing in batches of 1000
• using SimpleFSDirectoryFactory  (since Mmap doesn't  quite work well on Windows for our index sizes which commonly run north of 1 TB) 
   
  Looking at the thread dump, the bottleneck seems to be RealTimeGet and I can see that Solr 7 takes a different code path than Solr 8. Note that the performance of regular updates (non-atomic) is still pretty good on Solr 8 completing in < 1 hour for the same 20 million data set. 
   
  Sharing the indexing code, solrconfig, schema and thread dumps in the link below:
  https://drive.google.com/drive/folders/1q2DPNTYQEU6fi3NeXIKJhaoq3KPnms0h?usp=sharing