Depending on the traffic through your router there may not be enough processor/RAM to handle it. I had a 400 w/ 128 and it ran fine till i added havp white/blacklist then it got slow, more ram helped slightly but once i got my hands on a 1.7 w/ 512 it ran much better. the MD5 may also just be corrupt from the dl, or the device may have bad sectors. are you using zs 1.0.beta11?
to turn off the AVcheck simply uncheck the “enabled” choice under http proxy.