Speed · Plain-English answer
Buffering with a VPN usually means the path through your chosen server cannot sustain the video bitrate. Distant or busy servers add latency and lose throughput, and HD or 4K streams are unforgiving. Switching to a nearer, quieter server fixes most buffering, and your base connection speed sets the ceiling.
A video stream needs a steady flow of data for its entire runtime, so it exposes weaknesses that bursty browsing hides. HD wants roughly five megabits per second held continuously and 4K wants around twenty five, and any dip below the bitrate empties the player's buffer and pauses playback.
Through a VPN, that steady flow must survive the extra hop, the server's shared bandwidth and the route between the server and the streaming service. A server that is fine for browsing can still fail a sustained 4K stream at peak time.
Switch to a server nearer to you, or nearer to the streaming service's region if you are watching another country's catalogue, and avoid obviously popular servers in the evening. Confirm what your line delivers through the tunnel at /tools/speed-test/, then compare that with the stream's needs.
If your baseline without the VPN barely covers the bitrate, no server choice will save you, and stepping the stream down from 4K to HD is the practical answer. Wi-Fi problems also masquerade as VPN problems, so test near the router before concluding anything.
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