This problem seems started from 10.2-R, and the current X server is xorg-server-1.17.4 from 10.3-R.
Moving any X window around is very sluggish, and text message output to exposed xterm chews 100% CPU. For example, doing a large compiling from a xterm, when xterm in covered by another window, CPU is used only by the compiler; however, when this compiling xterm is exposed (can be seen), system becomes slow and top shows that xterm takes 100% CPU on one core.
Playing video on firefox does not have much performance issue.
It is not clear if this is a xterm issue or nv driver issue in xorg-7.7_x built under FreeBSD.
The xorg-7.5 (xorg-server 1.7.7 from 8.4-R) works very well with GeForce 210 card, and X from the latest Ubuntu release 16.04 LTS (xorg-server-1.18?) also work well on this video card.
Is this still relevant?
Have you tried using the official binary drivers from nVidia?