xhtmlrenderer optimizations :css taking too much time

Question

I'm using flying saucer xhtmlrenderer to generate pdf from xhtml. The issues i'm facing with it is that xhtmlrenderer is taking too much time to generate pdf when css files are large or there is too much css. I tried to generate pdf with and without css. And the results were as expected. It took nearly 3 sec without css and 50 secounds with css. Can anybody tell me some way to reduce generationn time. I tried to optimize my code but since xhtmlrenderer is external lib, i can't do much to optimize it. Any help is appreciated