I have a DS411j Synology NAS that is getting quite old… 10 years actually. At some point synology updated this with something called “universal search” that just slowed the whole thing down to a point where it was near unusable. I found this post online explaining out to manually remove it. It has helped a lot.
https://randumbthoughts.com/2017/07/19/revive-your-slow-synology-ds411j-nas/
John's Blog 