Result grouping performance

[Kempelen , Ákos]

Hello,

I am migrating our codebase from Solr 4.7 to 7.0.1 but the performance of 
result grouping seems very poor using the newer Solr.
For example a simple MatchAllDocsQuery takes 5 sec on Solr4.7, and 21 sec on 
Solr7.
I wonder what causes the x4 difference in time? We hoped that newer Solr 
versions will provide better performances...
Using Field collapsing could would be a solution, but it produces different 
facet counts.
Thanks,
Akos