2142's Titan mode eats your processor for breakfast. I'm not kidding; I went through a lengthy process trying to understand what the issue was. And lo and behold, even a computer with a sub-par workstation graphics card with a fast processor outdid my other gaming system in the game due to processor power alone. And the final upgrade to a whole new gaming system sealed the deal.
It's not due to "your computer being too slow," per se. It's actually more about how the engine is limited in some ways, where the programmers can't or won't provide workarounds to stop certain things from happening, which suck up performance. A few of these would basically lie around hit detection when the Titan AA guns and deployed sentry guns are firing. People constantly pound titan shields, trying to get aircraft taking off. That slows the server down a very noticeable amount, causing some major skipping and sync issues. And your latency/lag is left unaffected.
Then the sentry guns planted in hallways. They constantly fire at targets not even reachable (beyond a wall, for instance), with each bullet striking (and canceling out) the wall. And the target beyond the wall. And all walls past that point. This adds to the issue. Hit detection in games is a programming mess, if you ask me, but it has to be server-side instead of client to help prevent eliminate cheating and to set a middle-ground for people with varying latencies/ping.
Add to the fact that the Titans are in fact moving geometry/vehicles, the programmers have to use some workarounds for that as well. There is hit detection and not necessary the standard clipping practices when it comes to game maps, so you're effectively going to rely on the server determining where you are on the busy titan. Every footstep requires some form of synchronization with the server, so with many players, it'll put server-side load through the roof, especially with all the shooting.
And this all isn't even counting the client-side stuff (graphics detail, sound, etc). As far as performance is concerned, but that's generally quite solid, actually. Perhaps better than BF2 at times, however there is a bit more stippling in the trees.
Things get problematic when the server is having issues, so the game becomes unplayable. This happens on even high-end systems.