Can't access a specific site on the network. What should I check for?

[ITJosh]

On occasion devices cannot connect, or is really slow to connect to a specific website. This time, it is Salesforce. The site is not down, and can be reached outside the network just fine. What should be checked to troubleshoot this? I have http caching disabled, and I tried placing a network rule to set the priority to high without helping.

[CptnCrnch]

I'd possibly start by running Wireshark on the endpoint itself. Many things like slow DNS responses etc. are quite easily detectable from there. If there's nothing obvious, I'd go further down the network: are these clients located "behind" WiFi? Are they working via cable? Is everyone affected or only single users?