As I've said before, the line between microcontrollers and microprocessors is a fuzzy one - see this thread:
http://www.8052.com/forum/read.phtml?id=25129&top=

"a small number crunching program, say 58K"

"small" is a relative term:
58K is not "small" for an 8051; it would be small for an 80186/188, and tiny for a 32-bit microcontroller.

"requires as much speed as possible"

The 8051 is not really built for speed, nor for number crunching.

"figure the accurate speeds of running the program"

the datasheet will tell you the execution time of each instruction; it's just a simple (but laborious) case of adding them all up!
(You might need to do some statistics to cope with branching)

"Can software simulators run programs at the same speed a chip would run them at? Or does the simulators running speed depend on the desktop speed?"

Obviously, it depends on what you're trying to simulate, and the power of the host machine.
Usually, simulators don't even try to run in real time - they will tell you timings in terms of clock cycles, whence you can easily calculate actual times.