Recast graph not covering entire mountain surface

It seems no matter how I play with the settings, I just can’t get the recast to cover the uncovered regions atop the mountain. What am I doing wrong? And is there a way to manually paint an area with navmesh?

On a side note: my recast graph has a width and depth of 14k, and despite caching it, it’s bringing my framerate down from 250 to 160 fps. Anyone know why this may be happening?