Oh yeah, something more to add...
I seriously doubt USB Keyboards use more CPU resources than PS/2 keyboards...
Reason being: Both the Universal Serial Bus and the PS/2 connections require some hardware based encoding/decoding since they are serial busses (narrow ones at that). Hard to explain if you haven't engineered anything using USB or regular serial bus before...
The point is both require HARDWARE based encoding/decoding of the signals accross the line. It could be done in software, but that would be extremely unecessarily complex and costly.
If the case of mice and KBDs, each peripheral has it's only little internal IC that does all the signal encoding for transfer over the serial bus. Then, the CHIPSET does all the decoding of the incoming serial data stream. More specifically your south bridge chip (it handles all the peripheral I/O stuff, whereas the northbridge handles communications with the CPU and RAM) does the serial data decoding.
Either way, the encoding/decoding of the data streams are all done in hardware and should therefore incurr no tangible performance inpacts.
I'd be surpised if anyone could prove that the USB version of your keyboard slows down your system to any benchmarkable smaller numbers.
Although there is a bit more to it than that...
USB devices need software support, but then again, so do PS/2 port devices.
There is one way you configure your USB KBD to degrade performance slightly.
PS/2 keyboard software support is normally provided for in your BIOS. USB software support is either supported at the OS level or in the BIOS (assuming your BIOS supports it, which all new ones should, else you won't be able to use your USB KBD in Dos or the BIOS or anywhere but the OS). If you have the software support set for the OS, there may be a slight performance penalty. However, if you enable USB keyboard support at the BIOS level it should not have ANY performance differeance than when using PS/2.
BTW, in my ASUS K7V bios the setting is called something like: "Enable legacy USB Support" or something odd like that. Setting it to enabled or auto does the trick...
Oh yeah, just in case you haven't figured it out I have studied up on actually implimenting a USB peripheral on a simple device I was working on for work. USB's are a hell of a lot more complex than ordinary slow speed serial busses (such as PS/2). Fortunately implimenting them isn't too too much (I think, I haven't yet made one work yet) more difficult since we already have purpose built USB tranciever IC's that do all the bitstuffing/unstuffing, NZRI (I think that is what it is called) encoding/decoding, and all that other complex USB specific jazz... Overall though I like USB anyways, even if it is allot more complex than a slow serial bus... The increased bandwidth and standardization all makes it worthwhile.