What makes a watch run slow?

[BiG JeEzY]

I agree with what others have already said. Probably one of the main reasons your watch is running slow is because there is more friction being generated between moving parts. This is due to the possibility that the oils and lubricants are already starting to dry up.

All that extra friction also lets the balance spin slower because of the dried up oils. In turn, that extra friction leads to the whole movement working harder which lowers the power reserve of the watch.

That is my theory to why a watch runs slow. Try to get the watch regulated and if the watchmaker performing the regulations recommends a service, I say you should go for it.