A few days ago I had a revelation... the default settings cause the Eris to disconnect from wifi when the phone goes to sleep. Then, when the phone wakes up, reconnecting causes a few seconds of major lag. Incidentally, this is exactly the situation you are usually picking up or dialing out from.

I did this:

Settings>Wireless & networks>Wi-Fi settings>[menu]>Advanced>Wi-Fi sleep policy>Never

and so far I've noticed a huge improvement to responsiveness immediately after unlocking the screen. I was a little worried about battery impact, but it doesn't seem like an idle wifi connection is draining much more than an idle 3G.