I recently launched Minecraft 1.15 release version (December 11) and created a new world with default generation. My standard FPS in 1.14.4 was around 60 but in the update I got a maximum of 17 and only for a few seconds. The average FPS in my test was 2.

This screenshot shows my game debug information plus the debug pie graph and bar graphs(note the initial spike in the bottom left is when the world was loaded). The problem seems that 99.41% of my game's memory is being used for tick, with 0.59% left for everything else.

For Everyone who would suggest I change my graphics settings, this is what they were set to when the above screenshot was taken.

I don't know what the cause is so I can't help try to isolate the bug. I know that the problem doesn't affect everyone since my friend played 1.15 on a similar machine and got very good performance.