Bought a new PC before BfA - RTX2070 and i5-9600K. Way below the 2k$ mark. Only raid encoutner I ever had bad FPS was Zul, because I had everything on maximum then. Tuned down some stuff I can't see (or ignore) anyway - no bad FPS since.
If you're having bad FPS with a good system, check your addons, check your settings, check your drivers, check if any other program clogs your PC while playing.