Ok, got it.

change imap-login and pop-login to these like showed in dovocot wiki for high-performance login mode.

service imap-login { chroot = login service_count = 0 client_limit = 600 process_limit = 100 process_min_avail = 16 } service pop3-login { chroot = login service_count = 0 client_limit = 600 process_limit = 100 process_min_avail = 16 }