I made a program for it on my NXT in assembly that counts and displays the number.(It displays the number in the same place the whole time.) It counts to 10,000 in about 4 seconds.

there's no way that you are getting 2,500 frames per second. i'm guessing that whatever method your using isn't "turning off the OS overhead" like qwerty said, and that it manages screen updates itself with interrupts, and that all you're doing is writing the character data to a RAM buffer somewhere.
the processor in the NXT brick runs at 48 MHz (or so the internet tells me), and the z80 in the 83+ and 84+ run at 6 MHz and 15 MHz respectively, so it will, indeed, count faster. like qwerty said, that's not the best way to determine "how fast it will run," but when the differences are that obvious, it's not a question.

Also, the NXT has an ARM7 processor, correct? The architecture itself is faster than the z80 too (stuff takes less clock cycles, there are more registers, 32-bit instead of 8-bit, etc)

Yes, it has a 32-bit ARM7 Microprocessor running at 48 Mhz.
DJ_O-
Well actually, even if I make the program not display anything, it is faster than the TI 84 not displaying anything. And I actually think the NXT's display driver is faster than the Ti-84's. I can tell you the refresh rate. It refreshes every 17 milliseconds.