The 3DS should be able to emulate the GBA pretty easily. Nintendo won't tell us what CPU it's using, but rumors say it's got two 266 MHz ARM11 cores. To compare, the DSi has a 133 MHz ARM9 and a 33 MHz ARM7, the DS has a 67 MHz ARM9 and a 33 MHz ARM7, and the GBA has a 16.8 MHz ARM7. I don't know if the dual-core 266 MHz spec is right, but it's completely reasonable when you look at the strategy Nintendo's used on their previous handhelds.
Given that ARM CPUs are all pretty similar even between generations (and Nintendo has probably specifically tweaked the design to make them MORE similar to make backward compatibility easier), it's perfectly reasonable to compare clock speeds directly. So it wouldn't be an exaggeration to say that the 3DS is over ten times faster than the GBA. As a rule of thumb, it usually takes about 10x more CPU power to emulate a system with a different architecture, and the 3DS easily meets that requirement.