I need advice for better performance

Hiya folks,

We are running multi-site but we need better performance.Because our dashboard too slow.

I read a lot of articles for performance but I get some odd error.
For example when I enabled memcached and used object-cache.php some blogs get 500 error,and some content duplicated.
Also we are using quick cache,yes it is very nice for multi-site farm.
But dashboard still slow...

Please give me some secret / suggestions.

Thanks,

Pictures for our system usage