1. I don't do anything special to handle loading latency, but it doesn't seem to be too much of an issue so far. At each sequencer step I issue the pad command to the YT.Player. If that part of the video hasn't loaded yet, then it doesn't always sound right at first, but Youtube seems to handle it pretty well by caching "where" you've been in the video.
2. Yes, there are 4 sequencers. At each step, for each active sequencer, I call an executeCommand function. Right now, just in a for loop. At one point I tried doing this with a Promise but I think the overhead of creating Promises so many times a second outweighed any benefit in performance and I scrapped it.
3. The AWS free tier expiring is actually a matter of time rather than server load. I have essentially 10 more months of free EC2 usage left. Server stuff is just the pattern sharing which is pretty lightweight, just storing and serving text files. I would have just used MongoDB which would have been free but I wanted to learn EC2, GraphQL, RDS.
4. I don't have a Featured Patterns page but you can go to the Share Patterns page and sort by Most Liked. I do agree that it would be a good idea though, perhaps as the default option of the sorting dropdown. But first I need to get the word out so the community will actually make some rhythm patterns ;)