Depending on where you are located different sites are accessed through different routes that traverse various routers managed by the tier 1 telcos like MCI, etc.
It sometimes happens that a particular router may have a problem with a specific route.
This means that many, almost all websites will work fine for you but a few or even one will be sluggish for a while. They usually fix it themselves.
The troubleshooting process is beyond the scope of this forum, it starts with tracert.
If you have the relationship or authority to get the help of your IT person they can resolve it for you.