Sigma or Canon in the 100-400 range????

I've been looking at the canon 100-400L lens and started thinking about a Sigma in the same range because of price.

I have a Sigma 18-270 and found it is very slow in focusing for any kind of action shots. I went back to using my kit 18-135 and got better results.

Is this an issue with all Sigma lenses mounted on a Canon body? Meaning the slow focusing? I'm using a 7D.