1. Rear leg side-kick, powerful but too slow.
2. front leg side-kick, choose your oppurtunity wisely
Since you might have not trained the rear leg version very much recently, I believe your speed might be a bit off.
The speed in you kick comes from lots of practice.
I use the front leg version aswell, but my rear leg version will put you down sore, more often.
Also when used in close range, the rear leg version can easily be changed in an ura-mawashi-geri or an o-soto-gari.
I also like the 'man-stopper' to the inside thigh.
Speed can be learned by practice. Saying the rear leg version is too slow just doesn't cut it.
But hey, if your front leg version is fast and powerfull cherish it!
Keep up the good work!