Possible the AV scanner scans the IntallOptions.dll but also possible it's because you're using solid compression.
Try to reserve the file e.g.
this might eliminate the noticed delay.
SORRY, I MADE A MISTAKE HERE.
I found some of computers are using Mcafee AV 8.0.0(enterprise). Then the installer will get delay.
If the computer installed Mcafee AV 8.5i(enterprise). Then there is no dalay.