Ideal lightweight setup…

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Check your TVHE settings and your Jellyfin settings to see if transcoding is happening. Additionally, hard-wire the pi if it's not. The only other advice I can give is to ensure your OTA antenna is getting good SNR. My setup isn't perfect (I have a Hauppage USB tuner) but it's decent when I mess with settings -- in my case, forcing transcoding made sure everything matched up.