Streaming is primarily throughput-sensitive. Higher resolutions and multiple simultaneous viewers require more sustained bandwidth, while instability can still trigger quality reductions or buffering.
Consistency over peaks
A stable 30 Mbps link can be more useful than a link that briefly reaches 100 Mbps but frequently collapses.
Shared connections
Other devices, cloud sync and software updates consume the same access link.
Test close to the problem
If one television buffers but your phone does not, Wi-Fi coverage or the device itself may be the bottleneck rather than the ISP.