Hmmm, well, it's BEEN me! I typically say something like, "I'm sorry, but my walking speed is a bit slower" and leave it at that. My partner walks MUCH faster than I do, and I generally attempt to keep up, but when it's irksome, I comment simply, "I'm not going to walk this fast, I'll see you later" and that usually works.
From the other perspective, though, I've often been told that just as my natural speed is slower, others are faster and they cannot help walking fast any more than I can help walking slow.
It doesn't hurt my feelings, or seem rude when others are quicker--- and even if they don't seem to notice that they have left me behind. We're all different and I'm typically the one in the group who finds things on the ground, or notices interesting things that others fly past. I'm not even sure the "fast" people realize that THEIR PACE is the reason I'm a straggler---they may think I stopped to tie a shoe, or who knows what.