What model of Linksys router, if its an older generation, probably the onboard cpu cannot handle the internal network load.
For your setup I would employ a Gigabit switch, in this config.
ISP <--> Router <--> Switch <--> PC's
As your bypassing the router for internal traffic, things should get quicker, wireless traffic will be as befor or maybe a little quicker.
The router will still be handling DHCP and internet access.