There could be several reasons you might have when playing a video in your browser. The issue depends on where you play the video from:
Streaming a video
Most video performance issues are related to streaming.
You're never streaming videos from the Toolkit's servers - it's usually from a dedicated file server which makes streaming issues unrelated to Toolkit issues.
To reliably stream a video you need a good internet connection, the higher the video resolution is, the better internet connection you'll need (you can test your connection speed with any speed test site, for example: http://speedtest.googlefiber.net/). If the video you're working on looks like an high resolution video, you should ask your client for a lower resolution video to achieve smoother playback.
You might have a direct link to an AWS S3 bucket URL which means your location and how far you are from where the bucket server is matters a lot - in such case, you can request a CloudFront link instead which should work faster.
If the video is an MP4 and it's not protected by any digital rights, you should be able to use the link (if you have it) to download the video to your computer and play it from your local hard-drive.
Playing a local MP4 file
Playing a video from your hard-drive should be smooth. If you have any performance issues in such case, you should try a restart you computer. If you still have issues, they might be related to your computer's performance (CPU, GPU and RAM) and you might want to try from a different (newer) computer to rule that out.
Playing a local MP4 file with Ooona's Agent
If playing a video with Ooona's Agent isn't smooth, you might want to try and open it without the Agent to see if it's the bottleneck in the issue and if playing the video directly from your hard-drive solves the issue.