Backface culling enabled in retopo mode dramatically reduced performance

when enabling "cull back faces" my viewport frames drops from 120 down to 15-30 fps.

this is while retoping a mesh.

this is due to the increased amount of computation. There are a few options which can affect performance sharply. There is a page in the help file about this.

Help > Contents > User Interface > Viewport Performance.

I always thought back-face culling was a performance saver ? It's surprising to see perf takes such a sharp hit with it enabled.