Jet fighters are made to cruise at a high subsonic speed, even at a supersonic speed (supercruise?). It means that they can be flown comfortably at relatively high speeds.
Jet fighters are often used to intercept aircraft, whatever the reason (assistance, escort from national to international airspace, check an unresponsive aircraft,...).
General aviation aircraft can cruise at a slow speed compared to jet fighter. Moreover, if needing assistance, it may fly even slower. I imagine it is possible that a GA aircraft needing assistance cannot fly at a speed above the stall speed of a jet fighter. I know that some countries dispatch choppers to intercept slow aircraft to handle such situation.
I imagine the military doesn't fly their jets at stall speeds when close to another aircraft (a margin is needed).
Is there a speed below which a jet fighter cannot intercept another aircraft? Otherwise, how do they do it? (I imagine you can orbit above another aircraft but it is less convenient to guide it safely to an airport)