Fiddler offers many ways to filter data. The most powerful mechanism is FiddlerScript. Click Rules > Customize Rules. Scroll to OnBeforeRequest
and add:
if (oSession.urlContains("SignalR/poll?")) { oSession["ui-hide"] = "FiddlerScript hides signalR"; }
Save the file.
(It's not entirely clear that SignalR's longPolling requests really have "nothing to do with development of the website", but if you don't want to see them, they're easily hidden.)
Incidentally, the next build of Fiddler's Filters tab will include a Hide URLs containing option. Thanks for the suggestion.