"machines are still getting faster"

You should only compare machines that are actually comparable. You can't take a desktop/laptop from 2007, and compare it to netbook from 2008 and proclaim "computers are getting slower!". If you want to do such a comparison, you should compare desktops to desktops, laptops to laptops and netbooks to netbooks.

That said, I really can't see how performance of the computer could be an issue in a game like Wesnoth. Maybe for some really, EEALLY low-end computers, but should the goal of a game (any game) be that "it must be playable on every computer in existence, no matter how old or slow!"?