Slowness after moving to Port-Channel?

The setup was, internet line-> router<>switch<>firewall<>proxy.

Earlier, there was only a single interface between the router and switch and this had sub-interfaces.

I used 2 new interfaces to create a port-channel and recreated the sub-interfaces here. (Basically moved everything from the single interface to new Po)

Both old and new interfaces are Gigabit interfaces.

This happened some weeks ago, Now I have users complaining of slowness when connecting to internet and they say it started around the same time after the move. I checked the port-channel and individual interfaces and don't see any errors or drops.

Any clue as to what else I need to check?