[Dovecot] A large number of imap processes

My system is very slow, the Maildir is mounted via NFS, the index is on a local disk system.

In the config, there is:

max_mail_processes = 3000 login_processes_count = 6 login_max_processes_count = 1500

I can see a large number of processes named "imap", some users have more than 12 such processes running. Is such a large number per user a "normal" behaviour ?

What is wrong here ?

Thanks a lot ! Claude