By upgrade to pfSense, I assume you inserted a piece of hardware running it into the network. If so, then yeah latency will increase, as the packets have to pass through the hardware. Don't forget, that packet has to be received, processed and transmitted by pfSense, so it all adds up. Also, if you're still using that Linksys as a router, don't bother. Just use it as an access point & switch. That will remove the latency of the router portion. See what the latency is when passing only through pfSense