I don't think a lightmap of 1 is a good idea, It most likely will kill performance for many users and boost the map's file size.
This performance drop is probably due to the lack of optimization in Source, Not your PC, I could be wrong though.
Try 4/8, You shouldn't lose too much quality in visuals and you'll definitely get a boost in performance.
I once did a few tests and a lightmap of 1 absolutely killed my performance, Whilst 4 ran smoothly.