I'm on Mac OSX (HIgh Sierra) and I have noticed the timer is running slow. I've compared this to a manual stopwatch and it lags about ~10 by the end of the 25 minute session (so it's reporting I still have 10 minutes left when a full 25 minutes have passed).

No idea what's causing this. I usually start the timer, then minimize it while I work away. Haven't tried running it solo with nothing else running.