Any # of reasons. Edge versions could be different, the machines may be running through different gateways/switches, the site may rely on hardware acceleration and it isn't turned on, Edge performance settings could be different, one machine may have already been to the site before and so all the various caching and optimizations that browsers tend to do may cause performance differences.
I would recommend you use the Developer Tools in the browser (F12) and go to the network tab and force a refresh of the site on both machines. Then see where the differences are. Could be the paths to the various CDNs the site relies on are impacted by network routing rules differently. The Network tab will identify where slowdowns are happening.
As for your title question about XHR, that is a web request the client is making to a server, generally an API call to the site's backend server. Most sites make heavy use of this. If the call is slow then it might be a problem with the server backend. You'd have to debug this or contact the site administrators.