Well the greyed out figure in my image shows 15 seconds, you could try it at that and see if it makes much difference and increase if required.
Not sure if it would impact on an upgrade to win8.1. Whilst I don't have win8 on either of my systems, the upgrade to 8.1 is meant to be very large and there no doubt would be lots of executable files that would be being scanned, that could slow the upgrade and install.