One quick fix to try, if it's the login-time that's killing you, is to enable auth caching:
http://wiki2.dovecot.org/Authentication/Caching
that should offload your backend LDAP-servers from doing bind() on ever
login, had a huge login performance impact for us. We use
"auth_cache_size = 100 M", which gives us 99% cache hits:
dovecot: auth: Authentication cache hits 3654591/3669119 (99%)
dovecot: auth: Authentication cache inserts: positive: 588030
80931909B, negative: 912 49888B
-jf
