After installing a new drive and reinstalling 10.5.2, I set about copying key files and programs back to the machine. However, my AirPort performance was glacially slow: time estimates to copy 70MB of data were in excess of 45 minutes! Some Google searching told me that I was not alone. Apparently this is a pretty big issue for many people on 10.5.2, but it's not universal -- our MacBook Pro, for instance, sees no such slowdowns.
I haven't marked this hint as 10.5 only (as the solution can be implemented on 10.4, too), but it does seem that the problem is limited to those running 10.5.2, and using AirPort on either Intel or PowerPC machines. Read on for the solution...
That linked thread from Apple Discussions contains a potential fix. John Albergo theorized that it might be related to the TCP setting for "delayed_ack", and offered this one-line Terminal solution:
sudo sysctl -w net.inet.tcp.delayed_ack=0
The default setting is three; the above command changes it to zero until the next restart. (For a permanent fix, John linked to this blog post that contains a Startup Item to run the above command at each boot.)
I tested it last night, and it worked perfectly on our troublesome PowerBook -- AirPort speeds returned to normal immediately after running the command. Theoretically, setting delayed_ack to zero is a Bad Thing, especially on a slower network. However, I transferred over 250MB worth of stuff (both data and applications) last night, and everything worked perfectly. Note that others have reported that the fix didn't work for them, but many people are reporting success.
In my case, I've chosen not to install the startup item -- assuming Apple figures out the real cause of this problem and fixes it in 10.5.3, I don't want to have to remember to remove the startup item. The PowerBook is only ever restarted for software updates, so I've just put the command into a simple one-line script for easy running when needed after the occasional restart.
Ironically, this very fix was listed here in this hint from a couple years back (and even earlier, in the comments to this hint). However, those solutions didn't come up when I searched for a solution to the problem last night, because they're about Samba and Windows file transfer speeds, and don't mention AirPort.

