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"Incorrect CSS programming" is a bit vague - what do you mean by incorrect?
The main way CSS could slow down page loading is by using lots of external style sheets (i.e. significantly increasing the number of HTTP requests) or by using very large CSS files. If you're using just a couple of style sheets under 100KB or so then the slow loading is being caused by something else.
It's probably more likely to be slow, resource-intensive server (or client-side) scripts.
Use the Firefox extensions above to diagnose the problem. Let us know how you get on and what the problem was.