Does the number of computers affect an internet speed test?
Doing a speedtest on one of the computers while all the other computers are idle (or even better turned off) will likely tell you the true speed of your connection. Keep in mind that the likely minimum speed of your connection on wireless or hard-wired is over 8 Mbps (slowest would be 10 Mbps) so any computer will be able to use all the 8 Mbps if available. You’ll probably find when testing on speedtest.net that you don’t actually get to 8Mbps but to something close. If you’re not getting close and you’re on wireless, try using a hard-wire at least to test to see if you get close to 8. If you still don’t get close to 8, you’re not getting 8.
