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[The Niles]

It's mostly because of a limit SP2 places on the maximum number of connections your pc can make at once. SP2 restricts it to 10 connections at a time, but normally you'd be allowed upto 50,000!

Here is what I found on this,

Windows XP SP2 limits the number of possible TCP connection attempts per second to 10 from an unlimited number in SP1. This can affect performance on server and P2P programs that need to open many outbound connections at the same time. Notes - With the new implementation, if a P2P or some other network program attempts to connect to 100 sites at once, it would only be able to connect to 10 per second, so it would take it 10 seconds to reach all 100. In addition, even though the setting was registry editable in SP1, it is now only possible to edit by changing it directly in the system file tcpip.sys. Keep in mind this is a cap only on incomplete outbound connect attempts per second, not total connections. Servers and P2P programs can definitely be affected by this new limitation. Use the fix as you see fit.

Keep in mind I have no idea how reliable this information is, there are a lot of sites out there that print a lot of none-sence about MS for reasons of their own. Source

On a Dutch forum I trust this problem has been discussed and it does appear real. There are some hacks to lift the restriction but do not attempt this unless you know what you are doing.