I can import a ~400mb .capx file in to C3 in about 20 seconds on my machine. So if it takes 10 minutes it sounds like something's definitely gone wrong. Given there aren't many other reports of this it does sound like something specific to your system. Maybe try disabling any browser addons you have installed?
If you have memory issues, try using Chrome Canary - they recently fixed some memory leaks and from what we've seen from other bug reports, Chrome Canary fixes most memory-related issues. In due course it will make its way down to the Stable channel.