We have the same problem. Changing anything on the schedule screen is incredible slow.
We don't have a lot of items - just one morning and afternoon repeating item for each workday of the week, so 10 items in total.
It's been driving me mad, but I have found that switching the wireless off (yes, the server is wireless laptop so we can update anywhere

) while I update the schedule means there is zero delay, rather than the 60-120 seconds delay *for each button click* that I was getting before. Unfortunately I have to reboot the laptop to regain decent connectivity to screen, but this can be done quicker than a couple of button presses would take otherwise...
Rebuilding the server install worked briefly but bad behaviour returned soon after

FYI we're using Kaspersky, not Symantec, but I will test without AV when I get a few minutes to see how we get on.