I am not an application performance person, but I just opened 10+ pdfs and ePubs and tried all the actions that were getting me the spinning beach ball of death in Zotero 6 (like moving entries between folders and renaming folders) and I am now not getting the spinning beach ball of death and things seem much improved.
(I'm not seeing memory usage over 2 GB on my very old Intel MacBookPro, I can't remember peak memory usage before but I think it was higher.)