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
}