All Bolexes slow down a bit as the spring winds down, and may run a little faster at the start of a roll when the take-up spindle is not being forced to slip as much - putting less load on the spring motor - so especially for older models that have less constant spring motors it's not a particularly exact thing.
For a pre-war Bolex that may not have been used for decades yours is pretty close!
Did you check it under load (with some film or when holding the take-up spindle)? With the spring fully wound, and after it had run for 10 or more seconds?
A strobe on the shutter or claw is a good way to check speed if you have a strobe gun. Bolex originally supplied technicians with little strobe discs (similar to what you find on record players) that fitted to the external drive shaft, with upper and lower tolerances. I can"t remember what the tolerance on those discs was though, maybe half a fps at 24? That's only a 2 percent error.
A service is always useful if you can afford it, but the really old Bolexes are more collectors pieces than working cameras, so don't spend a fortune if it's just for a bit of fun. Adjusting the speed dial up a bit is fine if you just want it running closer to 16fps, no need to send it to Yverdon for that.