Buffering can be caused by bottlenecks anywhere in the system. That could be origination video servers, server connections to the internet backbone, the internet backbone itself, the ISP's connections to the internet backbone, the ISP's wide area network, the last mile and the household network or wifi. The most common causes seem to be the video servers and the household router or wifi.
Local wifi can be checked by process of elimination. Is it just on one device? Does it occur only when multiple programs are being streamed? What are the results of a speed test using a smartphone near the devices? Wifi pods can be useful to extend signals but they can also cause speed issues, especially if poorly placed. Rebooting the router, pods or wifi device sometimes helps.