Speed Test

I recently upgraded from 30Mbps to 100 Mbps 3 days ago but my several speed tests on different devices show 35-40Mbps speed. I have called tech support and they asked me to do the basic stuff like restarting my router and laptop, clear cache, etc but nothing made any difference. Finally she said if I can signup for a monthly service, she can have more experienced tech staff look at the issue. The problem is at Cox and it is ridiculous that they are asking me to pay for monthly service to get them look at the issue. Anyone can help with the speed issue please?