when to implement VLANs?

I’m administrator of a branch LAN Ethernet 100mb/s using 4 cisco cat2924 & 2610 router to connect to HO with about 65 POSs an PCs,and 2 NT server one of them is oracle application server,and the other is file and print server, the performance of the network is very slow, I checked the compability of speed and duplex between the switches ports & NICs and their situation were very good, (i tried all the possibilties 100 to 100, autoselect to autoselect..etc, and still very slow )so i don’t thinkthis is the problem, and the performance the servers is good, and my questions are, are there any methods to find out why is it slow, and is implementing VLANs could speed up my network?