The speed issue may not be a problem with the split itself, per se. It could be that your network is having issues with the data tranfer from from the network to the users' C: drive.
Also, take a look at your VBA programming and your queries. If you have intensive querying out to the network, or if your VBA functions consistently tap into the tables "out there," that could slowing you down.
Another thing in your programming and queries that can slowing you down is how efficient they are. For example, the functions DLOOKUP, DCOUNT, DMIN, DMAX, etc. may be easier to use than creating recordset objects in VBA and using them, but they are much less efficient. The same operation using DLOOKUP vs. using a recordset object is usually much slower in the former than the latter.
I had that issue, and what I did was avoid using DLOOKUP if I could help it. Also, I had a start-up routine in the front end's MDB make local copies of data tables so that when they queried things, it went MUCH faster.
Michigan Dept. of Environmental Quality, Air Quality Division