That's weird to say the least ! Thanks for the detailed symptoms by the way
So, according to what you say, MITM is in fact working since you're using a spoofed gateway !
Now, about it being laggy, that's normal : even when I had an optic fiber connection (soon to come back I hope...) it was laggy during the attack.
Did you go as far as entering a pass for FB ?

For the iptables --list, well, there should be since we create one ! Try to enter it manually perhaps. (look at the script source, or sslstrip's website)
Were you root when doing this ? (I guess yes, but who knows !)

Ps : "Dear journal, I now have been on a shitty connection for a week. It's becoming unbearable."