Migration assistant really slow

Wireless is painfully slow. Under the best of circumstances it's half the speed of USB and if you are not able to use 802.11n protocols it would be nearly 1/10 the speed of USB. That means 1.2 or 5 GBs/hour. If you have a lot of small files times will be even slower. You did not mention how much data you are migrating, so I have no idea whether 10 hours is too much time or too little.

Kappy is right. It's not "Migration Assistant" that's slowing you down, it's using wireless.

You can also use Migration Assistant with a firewire cable connecting the two macs or you can even direct connect an ethernet cable between the two macs (no switch or router necessary). The ethernet would be faster if both Macs support 1GB ethernet.

No. You can cancel the current transfer then start over using Firewire or Ethernet. Firewire would be better if both your computers have Firewire ports. Some newer MacBooks do not have a Firewire port.

I don't know what the amount of time is to expect because you haven't told me just how much you are going to be migrating.

If you cannot cancel the transfer normally then you'll just have to force a shutdown by holding down the power button until the computer shuts down.

I used Migration Assistant to populate my data onto a new iMac being restored from a Time Capsule. I think it was about 250 GB. The restore was performed over ethernet (not wireless) but I should point out that it's true gigabit ethernet (1000 BaseT).

The restore took almost exactly 3 hours.

I should probably point out some math before you jump to conclusions. Regardless of network or bus speed, it's not possible to copy data faster than the maximum read or write speed of the hard drives. Most hard drives are going to have a maximum sustained transfer rate of maybe 45-50 MB/sec (some may advertise around 80 MB/sec this assumes completely contiguous data. Throw some randomized head-seek time in there and that will quickly degrade to about half of the max theoretical speed.

If you divide 250 GB by 40 MB/sec, the transfer works out to about 1hr 45mins. My transfer really did take 3 hours which works out to an effective transfer rate of only about 23 MB/sec. I suspect this is probably due to overhead of all the traversal required to reconstruct the data from the Time Machine backup format.

If you were to do the same transfer over 100 BaseT (100 Mbps ethernet) then the network would become the new bottleneck and it would not be possible to get a transfer faster than 10 MB/sec (and more likely 9). This would have turned a 3 hour transfer into a about 7h 40mins.

But you're not even using 100 BaseT ethernet... you're doing wireless. If it's wireless G then the max theoretical speed is 54Mbps (5.4 MB/sec) BUT... it's half-duplex. You'd be lucky to get 4 MB/sec. At that speed a 250 GB transfer would take about 17 hr 20 mins.

With that in mind... you said you've been waiting 3 days? Your transfer is hung. I helped a relative do the same thing several months ago, but after telling him what he needed to do he ended up trying to do the transfer over wireless. The sustained traffic load on his sub $50 wireless router ended up crashing the router and he was making no progress at all. The router had to be rebooted NUMEROUS times before he ultimately gave up, finally put the two machines in the same room so he could directly connect them and do the transfer via wired method. Over wire the transfer didn't take long at all (maybe 30 mins. But he didn't have much data.)