From 10 to 20 frames per second at 1024x600. Expect a bit more at 640x400 (or 640x480).
Download my "Netbook Addons" for Serious Sam, extract the .ini files and put them into the game's "Addons" folder (which is inside the "Scripts" folder - create the folder if it isn't there).
Start the game and in the main menu, go to "Options" -> "Execute Addon" and then click on "Netbook-1024x600". If you don't like the performance then execute "Netbook-640x400".
How much I played
From 'Sierra de Chiapas' to 'The City Of The Gods'. And on splitscreen mode (but only me) 'The Lands Of The Damned' and 'The Great Cathedral'.
On Linux, if the game looks like this:
Open the file "PersistentSymbols.ini" (in the "Scripts" folder) and search for the line
gap_iDepthBits and set its value to
24 (instead of
SHOULD YOU PLAY IT ON A NETBOOK?
Because it's Serious Sam game. And it runs decently on a Netbook (though a bit slower than The First Encounter).