All right, so symptom-wise, I'll give it a shot. I unfortunately haven't been closely watching my cpu and local hardware, as I was under the impression that most in-game lag was network related, and was watching my network. I will watch my hardware in the next few days to provide more information.

I do get spikes in upload, but they are only sometimes associated with lag, and have never been close to reaching my bandwidth, so I think it may just be association and not a cause.

I have managed to walk in a circle with no problem, and spin with no noticeable lag, even while simultaneously spinning and moving around (using the WASD keys to navigate as I spin). There is some upload activity, but nothing my network can't handle.

Appearance:
Most of the time, my lag takes the place of the game deciding to pause mid-action, for anywhere between a quarter second (at best) and 6 seconds (at worst). I then effectively "teleport" to a new position in the direction I was moving when the lag started, often continuing to shield (if I was shielding) or charging an attack (if I was attacking, whether or not I was holding it down). To summarize, the game takes whatever I was doing, assumes I want to continue pressing those keys for the next few seconds, then shows me the results.

Locations:
The worst lag is easily present when fire is on the screen. Any time Red Rovers or Fire Vents come into play, Fire Vents being much worse, I become effectively incapable of controlling my actions, being in the lag state approximately half the time. I can somewhat adapt my play style by only charging attacks and shielding a lot, but I often become incapable of even a full 2 stroke attack with my Divine Avenger, and the Vile Striker and other fast weapons become completely unusable.

Another location where lag is present, but mostly manageable, is when multiple of the gremlin bomb dropping enemies are present. Other enemies that produce occasional lag are lichen colonies (the big red ones) when they dash, and the jelly king (only when he is pink and spinning, and very mild). The use of multiple of the haze-bomb series status bombs sets off lag as well, cumulative with their size and number. I'm sure there are other minor elements as well, but these are the most noticeable for me

As you may be able to tell, the lag is most likely worst in arenas and danger rooms because there is a potential for many of these elements to combine and produce a cumulative effect. Fire-based arenas become completely unplayable when the 8 red rovers come out, and my lag is definitely worst in that situation.

Other Potentially Linked Effects:
I occasionally am unable to see bombs that I drop, which ends up with one of three results:
1) The bomb goes off, I see the explosion, but was unable to see the bomb or its radius/timer
2) The bomb goes off, and I don't see anything other than the damage
3) The bomb acts like it never existed

The reason I think these effects may be linked is that I never experience them when I am dropping bombs in terminals or other situations with no enemies.

As to the arms of zombies? Their reach is uncomfortably long, but I think that's part of the game :)