Diego Milano had a good idea on how to measure the performance of Android emulator: start a shell session (adb shell), and run
cat /proc/cpuinfo to get the BogoMIPS:
Processor : ARM926EJ-S rev 5 (v5l)
BogoMIPS : 280.98
However, this BogoMIPS value is highly fluctuating. Different runs of the emulator result in different values, so it is important to start the emulator several times. Although the typical value for my ~2 GHz system is around 300 BogoMIPS, one run resulted in 38 BogoMIPS. Nevertheless, if we assume the BogosMIPS value is not flawed, we need to find reference values for mobile CPUs. The Wikipedia article on ARM architecture has some:
220 MIPS / 200 MHz for the ARM926EJ-S and 600 BogosMIPS / 600 MHz for an XScale CPU.
So, one could assume my emulated 300 BogosMIPS compare to a moderate 300MHz mobile CPU, which happens to be a quite sensible reference value. The speed in the emulator running on a 2 GHz would be around the same speed of an low cost mobile CPU. The iPhone with its ARM11 CPU at 620 MHz would be more than twice as fast.
To take this assumption further, one could create the rule of thumb to multiply the speed of your desktop CPU by 0.15 to get a speed of a corresponding mobile CPU. Like this, the emulation running on a CPU with 2, 3, or 4 GHz would be as fast as on a 300, 450, or 600 MHz mobile CPU.
Keep in mind that all this is based on assumptions and simplifications.