Why cant Solidworks deal with sketches efficiently?

Occasionally I get jobs where I have to deal with sketches extracted from DWG files, that consist of many lines including splines. In general I find Solidworks performance to be utterly dismal. Especially with splines, my workflow grinds to a halt, and sometimes I cant even complete jobs or need to break them down into very small steps just to get it finished, which is more work than it should be.

When checking my system performance, my CPU and RAM have plenty of headroom to play with yet solidworks operates at a snails pace, what gives? And how can I improve the performance? I have a reasonably good machine with a quadro card, it should be way better than it is.