slow client/server application

While running my Access application in a client/server environment, my users
regard the application as being very slow -sometimes it takes 30-40 seconds
to move from one registration form to another. The two databases (one with
code, forms etc and another with the users own recorded data with some
10.000 items) are stored on a server and MSAccessRuntime is run from 5
different clients.

Does anyone have a suggestion where I should start locking to solve this

Any suggestion will be highly appreciated.