[AccessD] Access Speed Issue

I know a few tricks on speeding my app up but maybe there is some ideas that
someone has compiled.

Without getting into too much detail:

I wrote an APP that support 4-5 people. 
There is a main form and two sub forms. They are related. 
All forms are bound either to tables are through queries.
Database is split with Tables on backend.
Have a bound, hidden form connected to the backend so there is always a
connection. (Tip I got from the group)
I have already created plenty of indexes (which helped with the speed
until..)

Issue is that with one person it works fine, but with two or more it drags
to it's knees. 

Maybe a record locking type? Should I set the locking at the table level
(Row level locking)? Any other quick fixes I should be looking for.

Maybe the PC that's acting as a server may need some tweaking (record locks
and such?)

I have to run for a few hours but will look at the responses later.

TIA!

John Skolits