When you look at the tables in the database, the linked tables have an arrow beside them.
Because you mention multi-user, I can just about guarantee that the data tables are in a shared network drive.
But, for speed, all of the static tables should be in the front-end MDB, not the linked in network drive, which is called the 'back-end' MDB
Static tables such as ones that resolve a location code to a location description. The locations never change so put that table in the front-end. Or something like the container code for box, skid, single, dozen and the description that goes along with it.
The generic answer to 'how does that work' is that MSAccess was designed to work that way, if it's a multi-user setup.
MSAccess is not slow, I have an application that has a main-data table with 300,000 records and no report takes more than a minute. A query to find a set of records, say for a person, within those 300,000 records will return the set in 11 to 20 seconds. no records found is 11 seconds, 200 records found is 20 seconds.