Version:           Git (master) (using Devel) 
OS:                All

If I look at a folder in KMail2 with 15392 mails it takes less then a second to show the messages in the messagelist if i have threading disabled (Flat Date view)
When I look at the same folder in threaded view Activity by Date, threaded it's also nearly instant.
But if i use Recent Activity, threaded it takes 22 seconds before the messages show up in the list. Meanwhile the counter slowly counts up the threads to 6800.
CPU goes up during this time.

Reproducible: Always