The "cost" of high quality videos is big files, so the short answer to this issue is your internet connection may not be fast enough to stream the resolution you have selected in FulGaz, so you may need to lower the resolution you use for streaming, download the rides. More detailed answers to common streaming related questions are below.
"But I have super fast internet"
When you signed up for your internet connection you may have noticed an advertised speed. The real speed of your internet connection at the time and place where you train may be substantially lower. While the connection to the house may be fast, the wifi signal to the place you train may be very week, or increasingly, other people in your home may be streaming video themselves or using file syncing services such as dropbox.
To separate advertised speeds from your real speed, it's a good idea to download the "speed test" app and see what the actual download speed is in the place where you train. Again the time of day can play a big part in this speed reading, not just because of usage in your home, but in the buildings around you.
"But Netflix and YouTube work fine"
Streaming websites usually use a technology called "Adaptive bitrate streaming" This chops each video into sections a few seconds long, then delivers video at the best resolution it can for each section, depending on your internet speed. Even if you specify 4K, it may end up sending a much lower resolution at times if your internet connection can't cope. It's the reason why Netflix etc can look very pixelated at times. Because the speed of the FulGaz video is varying every second based on your power output, adaptive bitrate streaming doesn't work because each new section attempts to start at the real time, not the FulGaz time (faster or slower than real time).
"Zwift works fine, why doesn't FulGaz?"
If you download the videos first, FulGaz can work without any internet connection at all, but if you're streaming, it needs more bandwidth that Zwift (which does not work at all without Internet). The reason for this is the images in Zwift are 3D computer graphics, generated on the device you're using, it needs the internet to send and receive data about you and the other riders. This is less than the bandwidth needed for high quality video.
"How fast does my internet speed need to be?"
4K requires about 5 times the bandwidth needed for 1080p, and almost 10X the bandwidth needed for 720p. To add to the challenge FulGaz faces when streaming, if you tend to go faster than the original rider, you're effectively fast forwarding the video all the time, so you'll need a faster connection than a slower rider. Ideally you need an internet connection that is double the bitrate of the resolution of file you wish to stream.