The mech is a mesh much like any other, whether it be an npc, a human similar to that in the WIP now or a mech. True the mechs do have a few thousand triangles more then what "most" humanoid characters do, but as far as being a substantial drain on server resources that is changing. However, rulesets may prevent players from running around a 400 ton mech through downtown, I mean, no one wants to have to repave the streets every time there's a battle, i'd assume. So there may be some limitation as to the rules for where one can pilot a mech, i.e. you don't pull it up to the local bar and tie the reins on the post. Perhaps there may also be limitations depending on hardware however full LOD will be implemented in the code in the very near future. The social WIP as it currently sits, really isn't a good judge of this type of performance, as not much has LOD in effect, however as you see below, there's been some improvement.
The new implementation of the 1.5.1 server code dramatically increased both server stability and performance (FPS) in so much as some worlds currently listed in the WIP for MV by upwards of 200%. That's substantial, given what we've been working with in the past. I noticed a dramatic increase in stability myself in the closed offline test world, where I run both the server code and my client code on a vista hp laptop with 2gb of ram. I ran the code steadily for appx 12 hrs after I upgraded with no instability whatsoever in my test client, not one exception (which is a big change for my working environment over the 1.5 codebase). For performance (fps wise) I am not tops on performance, but I never have been when working in my closed test environment with my current hardware limitations (i.e. a laptop) and system requirements. However, I did see noticiable improvements in the WIP code over my previous code with an FPS increase from 20-25 to 30-35.
For me, I could really tell the difference in a test world in which MV Fantasy realm runs w/ test mob spawns. I used to see regular stutter in their target and combat rich environment, now however I no longer see that to a great extent....even though their test mobs still slide on death ala swg style.