What model of Synology?
Most Synology NAS have low power CPU, such as ARM or Celeron. Such units may have trouble transcoding video. Units with Celeron processors can use the embedded GPU ASIC (hardware accelerated transcoding) to assist with video transcoding. However, a Plex Pass is required to enable that capability.
You can review the NAS Compatibility List and associated NAS Compatibility Guide (Google Docs spreadsheet) to see the level of performance you can expect from your NAS.
Regarding moving from Windows to the NAS:
See this support document:
Move an Install to Another System
Suggest you read these two documents as well. They will help you avoid common pitfalls when installing & running Plex on a Synology NAS:
Installation and Setup of Plex Media Server on Synology
Synology Frequently Asked Questions
If your NAS has the Celeron J4xxx CPU (some 2020 models) and you do acquire a Plex Pass to enable hardware acceleration, then you definitely need to see FAQ #26. Otherwise video transcoding will not function correctly.
