One word. Bandwidth.
In a very cartoon like example, try fitting a tube of chapstick through a drinking straw. That would be a slow and painful process. The same is the case when you have multiple computers on one network. Depending on your situation you can do a few things.
Do you know if anyone downloads alot? If someone at the other computer is downloading music and also on youtube, then you may not even load google.com fast. I have had it happen to me.
1. Call your Service provider and upgrade to a higher speed.
This will most likely cost you more per month.
2 I cannot assist with this, but Set bandwidth limits per IP on the router. Each PC, like the houses in your community have an address.
They have addresses like 192.168.1.1 with the last number being the actual Computers number. You can limit how much they can use so that every PC shares.
It can get to be a headache if you arent familiar...but a simple start would be to ask if anyone is downloading anything during those times of slow speed.