I’ve been reviewing several Intel Bay Trail, Cherry Trail, and Braswell mini PCs in the last year or so, and I always end up recommending Microsoft Edge browser over Firefox or Chrome for people wanting to watch YouTube videos, as the last two browsers always drop many frames with the video stuttering regularly. However I noticed that while Edge is playing MP4/AVC (H.264) video, the other two browser would normally stream WebM/VP9 videos, and it could be the cause of the problem as H.264 can be hardware accelerated, but VP9 not, and the low power processor might not quite powerful enough to handle 1080p VP9 video decoding smoothly.

Source: Improve YouTube Video Playback on Low Power Intel mini PCs by Disabling VP9 Support in Chrome or Firefox (CNXSoft – Embedded Systems News)