Main problem of a shared Access database is that each client is modifying the physical database file(s) directly. That means for example that when one of the clients wants to update record, it must make a lock on that record - writes that information into a file (slow operation, particularly when its done over network) for other clients to acknowledge it, writes the change and releases the lock (another disk operations over a network). Whreas when this operation is done on databases like SQL Server, only commands are sent by clients over the network, and the database system in question handles the operation - and it is WAY faster because on its end all operations are made directly, not over network and can be far better optimized (eg. locks/records stored in memory).
What you can do to improve performance:
1) move the database to SQL Server if it is possible - really, you save yourself alot of trouble
2) the faster the network it runs on is, the better - for 30 clients 1000Mbps is a must (it also depends on database size)
3) if any client accesses the database over WiFi its BAD - it slows the database down, and risk of database file corruption increases alot - again; the bigger database file, the worse
4) index for performance
5) do "Compact and repair database" often - at least once overy month on databases >100MB