I never tried any planets larger than 800, so I can't give an answer. 15 FPS? You may want to post your Dxdiag so that I can diagnose the low FPS problem. You also will want to check how much RAM you have on your machine.
Hmm...there's nothing out of the ordinary I can find in the Dxdiag. Drivers are up to date, you have 16 GB of RAM (which is awesome), and you have a 4.0 GHz AMD 8-core CPU. It's probably the planet size and type that's causing the low frame-rate. But here's a way you can report the issue: https://forums.uberent.com/threads/how-to-report-issues-where-the-game-is-running-slow.60973/ Follow the instructions listed and post your LobbyID, UberProbe file (zipped), and screenshots of the problem.
Earth planets are extremely heavy because they might have hundreds of thousands of trees on them. Try to play on Metal planets for example and performance will be 3-5 times higher.
If you want to play on a planet of that size, try raising water level, it'll knock out most of the trees.
