The latest update to Xcode 6.3 includes Swift 1.2, latest reports suggests a significant improvement in speed.

Rather than feeling like using a computer that’s 10 years obsolete, algorithms that were borderline rate limiting running in the main UI thread just happen like they ought to. As a reality check, I re-ran the horrendously underperforming algorithm that I complained about awhile back, and rather than taking 320 seconds to calculate 7 log P values, it now gets the job done in 30 seconds.

The latest entry on the Swift blog also describes means to improve performance.