I read somewhere that speed increases how fast you can swing your weapon. Is it possible to fortify speed to ridiculous levels, making even someone who uses only their fists able to deal a ton of damage per second?

(In a non-modified game) attributes can only be fortified up to 100.

I found, once, an enchanted item which fortified my Speed to over 90 very early in the starter dungeon. The rest of the dungeon was essentially a walkover.
Ever since I have tried to start my characters with as high as Speed as possible (75 + dice roll).
But....to deal out damage you also have to be able to "hit" your opponent, that's Agility.

Ok, thanks. I thought it was possible to raise it up to perhaps 200, because I was able to get my endurance up to 99 with a high elf, then raise it past 100 on my first level up. I wasn't sure if it was normal, or perhaps it might've looped back to 1.

Not sure about fortifying through enchantments, but I've played many characters that become a were that automatically fortifies many attributes, including speed and hand to hand. Of course, I would unequip any weapons, hold in the attack button and just start moving the mouse around like crazy and watch most enemies go down in seconds.

Well, if I remember correctly from my last gameplay, fortifying Endurance before the levelup screen give one a better chance of getting more health points.