Is it possible for XP's connection sharing to be uneffective?

I have an ADSL connection and a LAN consisting of 2 PCs (1 PC with two network adapters) and it seems so. Although I'm the one that has direct Internet connection, I've got an impression that my connection is up to 10 times slower than with second computer disconnected.

Could purchasing a switch make things better?

Droid

purchase a router not a switch

Golly gee wilerkers everyone! Learn to Internet Search

if you get a router you will have no problems at all.
Connection sharing is not preferable at all. It is possible that if the second computer was using all of the bandwidth of the connection that the other computer on the network may not be able to use its fair share of the bandwidth and so it will seem slow. Though with the second computer on but not using any internet bandwidth the connection on the other computer(s) should not be slowed.
Using a router will address this problem and provide each computer with an appropriate share of the bandwidth relative to what they require and what is available.