Regarding the "lag", low performance, general choppiness, etc.
This is ALL down to poor utilisation of the rendering thread(s) on the CPU that are feeding the GPU - meaning your GPU is sitting idle waiting on the CPU 70% of the time. Probably because the game is busy doing something else.
This means that when a lot is going on the GPU isn't been fed sufficently and the game goes to hell... its not actually network lag, etc. even tho it feels like it.
There is no client side fix - tweaking cvars, driver options, etc. etc. might marginally mask it a bit but nothing will completely fix it til the devs get off their arses.
I'm a little worried about this as the symptoms are identical to a nasty, but fortunatly very rare, bug with COD4 - thought to be related to IRQ sharing/system bus saturation somewhere - it only affected a small number of PCs but required major hardware and software changes to fix - i.e. new motherboard and GPU or a new PC would usually sort it out, but its unfixable if your hardware/software configuration happens to be affected by it. If this bug is now affecting a large number of the player base then essentially the game is screwed because the only fix would be the slim chance that a new PC or major changes to your PC resulted in a configuration that wasn't affected - given the large number complaining about it thats a slim chance... the other option being a major rewrite of the game by treyarch.