It isn't clear, but from the subject, I suppose you export for the Web, not as application.
1.5.1 web export indeed exports applets, ie. Java compiled code. Despite some current prejudice, Java is a fast language...
2.0 web export outputs JavaScript code, and despite big progress made in JS engines, it is still a slow interpreted language... So no wonder the performance decreases.

" Can I go back to exporting .jar files in 2.0?"
Check the list of tools, I think somebody made an applet export tool.

Thanks for the response, PhiLho. I'm not exporting for web, this is meant as a standalone application. Basically I'm doing some scientific simulations where I need a graphical output, and I'm developing different algorithms testing for speed. So far I've done everything in 1.5.1, but it'd be nice to be able to use the retina capabilities in 2.0.

I just tried the applet exporter tool, and that seems to work in terms of exporting an applet in 2.0, although it doesn't support retina graphics. I'll send the tool maker an e-mail about that.