Help for multisite slowness problem, please

Hello,

I’m running a multisite web with hundreds of blogs and heavy traffic. It’s running on ngix with varnish on top. Problems arises when users are logged in (their admins are running very slow when traffic is high) and specially when they upload an image.

Each post have many images so file folders are full of them. I don’t know if that may be the reason of the slowness once the “upload image” button is pressed down.

I would really appreciate any help in this sense in order to speed up admin and specially images uploads. People in charge of servers are making many changes but nothing seems to solve the problem.

Thank you in advance