Deplete the reserve vacuum in it by pushing the pedal several times without it running and see if it's the same stiff feeling. Start it and see if it recovers. Check the hose to be sure it's not leaking or split. Pull it off the booster and see if it gets good vacuum (engine might stall). I'd also pull it off the intake manifold and inspect it thoroughly. Put it all back together, check for leaks (use the carb cleaner method), keep driving it and monitor for a repeat. If it happens again, I'd replace the booster.
I'd replace the check valve before condemming the booster first. Cheap part and easy to replace prior to replacing the booster.