I think I've found and resolved my problem with Plex running slowly. Turns out the Synology was configured for automatic hibernation after a set period of inactivity. The slowness seems to have been the discs spinning up after such a period. Since disabling this option, the delay and initial jitter are gone.You're right, that should be more than enough I think. If you're experiencing slowdowns and pauses only on the HTPC and everything else plays back fine I would look into any AV software you have loaded. You may want to try and exclude those drives or folders that contain your media from scans.
I'm not very familiar with Synology either. I do use one at work as a backup target for my desktop, but I rarely have need to read from it.
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