Is it just me or is squid cache slower when on more than one interface?

[W4RH34D]

We have a LAN and guest lan on different physical ethernet connections along with separate subnets.

When selecting LAN and OPT1 as interfaces in squid, things work ok.  When I just leave squid on for LAN squid seems to scream in speed.
We're using captive portal on each interface with a DHCP, and I'm not sure if that is the culprit or what.

Anyone come across this type of thing before?