Whether your video is 1080p isn't the concern, since it's the video codec that determines if your client device can play the file directly. If not, it may require Jellyfin to transcode it using software capability only (as you did not enable hardware acceleration).

For hardware acceleration, you should read the official documentation. Say, some Intel CPU supports Intel Quick Sync so you don't necessarily need a GPU.

Does streaming work if your on your LAN?

Do you have a bandwidth limit set for remote connections in Jellyfin? It's on a per user setting and labeled "Internet streaming bitrate limit).

What sort of bandwidth are you able to get through Tailscale?

That weird I am streaming videos no problem, check ur cpu usage while streaming on server

can't be bothered to look up what kinda hardware that thing has, but I'd venture a guess it's hella faster than my server - a core2 duo from 2007. that thing has no issues streaming 1080p between continents with months long uptime and zero hitches, over wireguard. the only thing I changed from the default is turn off transcoding.

What if you deactivate transcoding?

I would suspect something with tailscale, maybe double check settings or try wireguard

I love my Synology but it really doesn't have enough CPU to run Jellyfin

Check the admin jellyfin dashboard on your phone while you are streaming, does it say the audio or video is transcoding?

What device are you watching on, and is it happening to all videos equally or does an MKV lag more than an MP4 for example? Are there any errors showing in the jellyfin logs?

I know this, have been prepping for system design interview for some time. One word: CDN A strategically placed geographically distributed group of servers that speeds up the delivery of web content by caching it closer to users