Somewhat (but not to be ignored) it depends on where you live, as PAL standard videos (for example here) are uploaded (produced and displayed) at a rate of 25 frames per second. Therefore for any smartphone that doesn’t follow this rate at 1:1, 1:2 or any other rate that divides or multiplies easily (like 100Hz÷4=25FPS, when external powerbank attached of course) your eyes need to do some math (easy if not for extended time of usage, secondary would be calculating some of actual distance to the used screen size, etc. 
). And, my two cents here apply to any smartphone connected with YouTube stream (or any other currently watched one), nothing to do in particular with the Librem 5 (but things, up to your well grounded question or good reasoning question deserves/needs to be taken into account/control, even without battery life time compromises, IMHO, by using some software switch to 60Hz or even 50Hz, otherwise 30Hz will do for most cases scenario, like phoning/messaging cases scenario).
@kate.mason, you can check frames per second of any video by typing:
$ youtube-dl -F 'https://www.youtube.com/watch?v=XYKYXXXXXXQ'
EDIT: My above comment: “100Hz÷4=25FPS” isn’t related to Librem 5 screen (technically not at all) likewise one, related one would be: 30FPS×2=60Hz, and would at the same time mean that the preferable Librem 5 screen refresh rate of 60Hz is the only one to be used for long term watching of 30FPS YouTube videos (such 60Hz setting should be easily doable for Librem 5, I guess).