with this plugin, you've got to take it slow when you're tweaking. I've busted my site a few times doing too many tweaks at once.
it's a pain in the butt, but if you're seeing a slowdown on your site, you can even check out http://gtmetrix.com which gives a very detailed breakdown on what's causing the issues and exactly which scripts/files are doing it so you can go back and do more tweaking.
biggest advice is that you do the tweaks in preview mode and refresh your site after each change so you can see how it gets affected.
at one point I had set my stylesheet to be minified but that really slowed my site down, but it might word for yours. each template is so different so it takes some experimenting, but if you can get into a 'compulsive obsessive' mode for an afternoon, it's pay off.