Squirrels are quick little beggars, irl and in game. This is not a problem as they are also stupid and trap themselves in trees.
I recently had one in my homestead, up a tree. I tried an arrow, nit expecting to hit but I hit its left arm and was stuck there, I threw a few rocks and managed to knock it out of the tree and broke its left leg, it was however on the wrong side if a clump of trees so I needed to walk around the trees to finish it off. I tool one step and it crawled away 5 squares, I was still blocked but tried throwng something to slow it down, I missed and it was then another 5 squares away. Inspecting it shows a puncture in the left arm, a broken leg leg and a minor bruise, but it could crawl 5 times faster than I could walk, it did not try to climb a tree but two turns later it had gone. I could zoom out with the message 'there is activity going on here.'
A couple of days later it turned up again, but was right at the edge of my vision, I attempted a long shot and it fled, it did not try to climb a tree. It returned again a few days later, ut still had a arrow in its arm but the broken leg had healed, that did not help it as it was close enough for a northern spear to drop it in one go.
It seems to be a bug that have two limbs seriously injured does not affect it speed, it only seems to prevent it from climbing trees. The few other animals I have actively hunted do seem to slow down as the injuries mount.
Another bug is that in five days a squirrel, with no bandages or splints, but perhaps access to herbs, can cure a broken leg. I had one in game last year and it took weeks to repair, irl it could take months. This seems to be a 'feature' for all animals, I recently had an elk serious wounded with an arrow and a broadhead arrow, I could follow the bloodscape trail easily until I lost it some thick forest but three day later I saw it again, it was the same elk as it had the arrows still but was now completely healed.