There seems to be a memory leak related to resource packs. The more times I join the same server (which sends clients a resource pack when they join), the more memory usage that Minecraft has and, therefore, the worse it performs.

This issue seems to also occur in 1.14 (I couldn't add it the affected versions list, it only shows 1.15.1). Here you can see how, after joining a couple of times to the same server, the memory usage goes up and the client is hardly able to get 5fps.

Video timestamps:

 - 0:30 - ~40 fps after loading chunks (first join).

 - 1:20 - ~25-30 fps after loading chunks.

 - 2:40 - ~15 fps after loading chunks.

 - 3:35 - ~ 3 fps after loading chunks.