After upgrading to XtraDB Cluster 5.7 (from 5.6), we noticed a severe performance decrease for select count(*) and sum() queries. Turns out there are several bug in 5.7, and some are already fixed.
For example Bug 81854 (https://bugs.mysql.com/bug.php?id=81854) has been fixed for the 5.7.18 release (that will hopefully appear soon).
Would Percona push these bug soon after being published? With not the newest Percona version is 5.7.14