A VPN changes the route and endpoint visible to parts of the network. Playback may improve, worsen or stop depending on capacity, latency and service policy. A difference is evidence about the path, not automatic proof of intentional throttling.
Compare under similar conditions
Use the same device, programme and time window. Note whether quality changes or the player switches servers. A VPN test that also changes several app settings is difficult to interpret and may produce a misleading conclusion.
Understand the added dependency
The VPN introduces another service and its own connection quality. Its busy server or distant location can become the bottleneck. Encryption does not manufacture bandwidth or make an unavailable media source available.
Respect applicable access rules
Use only services and networks you are authorised to use, and follow the streaming provider's terms. A VPN does not establish content rights or guarantee privacy from every party involved in the session.