OH on Synology getting slower and stopped now

FWIW, I wouldn’t run on ANY NAS.
One of the two main problems of doing so is that there’s very few people to have the same setup thus very little who can (potentially) help with your problems.
Official docs also recommend to use a dedicated system.

To begin with, read
How to ask a good question / Help Us Help You - Tutorials & Examples - openHAB Community