Auto Focus performance with AF assist beam?

Just got my first speedlite (YN-468) and found the AF in low light to be slow.I don't have any lenses that are known for thier low light performance when it comes to focusing and was wondering if AF performance with a flash is most dependent on the lens,flash,body or
a combination of the three? Thanks

Not sure about the XSi and the YN-46 combo. Your 50 should be a champ at low light focus with the AFAB. I know my 40D with a 580EX and a 24-105 f4 will quickly and accurately focus in complete darkness in most cases (assuming the AFAB is firing.)

I would assume that the combo of the three you mentioned is going to determine your performance factor. Fast lens, fast body, good AFAB = top performance.