Hi all! I'm new to the forum (and to Porsche ownership

)... I purchased a 1999 Boxster (85k miles), great condition. I've been driving it for a few thousand miles now and recently noticed my brake pedal feels off. When I press the brakes, the pedal doesn't come back up as fast as it should. In fact, sometimes it feels like it's still depressed and I have to stick my foot underneath to raise it back.
My mechanic said he thinks it's the brake booster. Does this sound right? Would I be noticing something different if it was the master cylinder vs. the brake booster?
I'd hate to dump money to find out that wasn't the right fix! Thanks so much!!