Firefox mobile?

Very odd: on my computer it never uses more than 1GB of RAM. As for the limitations of your first setup, that sounds like a bottleneck when it comes to transcoding, which can occur because of a lack of GPU or CPU power, depending on whether or not you had hardware acceleration enabled. I would expect it to run much better on your new setup, but the RAM thing is worth investigating. I was running Jellyfin on 12GB of RAM until recently, and never hit 100% usage.