Hi Rowbo! I watched your slither.io video and maybe I can tell you something.
Starting from the length I know that many people say that at the beginning the shorter the better but less than 5 mins seems a bit too short in my opinion but that's it- just my opinion.
Audio and video quality were good enough (maybe the game seemed a bit laggy but I noticed a lot of people have lag issues with slither.io). I noticed the memes you put and they were well put, maybe there were a bit too much but the right time and placement will come with time.
The outro card is a bit too long in my opinion because considering the video length 30 sec of outro are a bit too much so you should reduce it a bit, in that case you have some more time for the video itself. I noticed you are young so you have so much time for improving!
Also the SEO part needs improvement (Search Engine Optimization, aka tags description and title). You tags are a bit few and generic so try to use some keyword generators and the youtube search bar (by typing the game and seeing the results, that are the most popular searches and you can use them as tags) or also try to think about what you would write for searching your video (something I'm still learning).
Well I think i don't have anything else to say. Bye and keep improving!
EDIT: You should also change your channel page layout by clicking the cog next to the sub button and create all the sections (videos, playlist, discussion, about) because that can give a bit more of a professional look!
This helped alot and i will use some of this in the future. 