The most likely reason for this issue is a lack of processing power on your device. Ubuntu 15.04 is an older operating system, and may not be able to handle the high demands of streaming video. Additionally, if you are using a low-quality internet connection, this can also contribute to choppiness.
One solution is to upgrade your hardware, such as getting a faster processor or more RAM. Another option is to switch to a newer version of Ubuntu, as newer operating systems often have better compatibility with streaming video. Additionally, make sure you have a strong and stable internet connection.
Yes, you can try adjusting the video quality settings on YouTube. Lowering the quality may help with choppiness, as it requires less processing power. You can also try clearing your browser's cache and cookies, as this can sometimes improve playback performance.
There have been reports of conflicts with certain browser extensions or plugins, such as ad blockers, that can affect the playback of YouTube videos. It may be worth disabling these extensions and seeing if the choppiness improves.
It is possible that using a different browser may improve the playback of YouTube videos on Ubuntu 15.04. Some browsers may be more optimized for streaming video than others. It is worth trying out different browsers to see if one works better for you.