Sometimes after a large file transfer (typically caused by T.M. doing a ridiculously large wifi backup in the order of Gb's) the transfer will slow down after start or subsequent transfers will become ridiculously slow. This is a bug in the TCP protocol and is fixed by changing the delayed_ack property in sysctl.config from it's default of 3 to 0, using terminal.
Type sudo sysctl net.inet.tcp.delayed_ack to check what's already present
Now type this sudo sysctl -w net.inet.tcp.delayed_ack=0 whereupon you will be prompted for your password.
However, this will not persist across restarts and editing the file may result in an overwrite in subsequent OS updates. Therefore, install a startup item instead. I've simply added 2 lines to my automount airdisks script (the second line closes the pswd timestamp so you don't have to wait 5min before the system asks for a pswd again - otherwise any unauthorised use of the system within this timeframe will be allowed WITHOUT being asked for a password):
do shell script "sudo sysctl -w net.inet.tcp.delayed_ack=0" password "********" with administrator privileges
do shell script "sudo -K"
Also, try this (unlikely, but might fix things):
Check your DNS settings. - System Preferences/Network/Airport - Advanced/DNS.
ping it to make sure it returns at a reasonable speed.
if the numbers are grey, that means your router is giving it to you automatically. you will have to change it in your router.
many people seems to have had success with using OpenDNS - I didn't use it myself.
Turn off IPv6 - System Preferences/Network/Airport - Advanced/TCP/IP.
Check your preferences. If you upgraded to Leopard, it can corrupt these preference files. These are in /Library/Preferences/SystemConfiguration.
com.apple.airport.preferences.plist
NetworkInterfaces.plist
Preferences.plist
Removing these will automatically recreate them. These fixed a lot of my weirdness on my system network. For people running Parallels, make sure it's running and open System Preferences/Network and it will recreate it. The only thing is you need to rename it. Look in the icon next to the '-' sign.
Parallels Host-Guest
10.37.129.2 255.255.255.0
Parallels NAT
10.211.55.2 255.255.255.0
For people seeing slowdowns in Safari, look in "username/Library/Preferences/com.apple.Safari.plist" - this also gets recreated when destroyed.
For those who have their older backups, you can go back to an older version of your /System/Library/Extensions/AppleAirport.kext
This is not OS X's fault but you should try changing the wireless channel in your wifi router. I went from channel 6 to channel 11 and it helped a lot.
