Wesnoth makes current hardware sweat to the point where my relatively new MBP sounds like it's taking off, as is my friend's ThinkPad. It's also the only game capable of slowing my current-generation iMac down to the brink of operating system usability and beyond.
The game itself is slow, UI actions, moving units across the battlefield - pretty much anything happens only after a noticeable pause. There is no apparent reason why a largely static, 2D, tile-based, pixel game should be this slow.
The network code and its underlying model are an unmitigated disaster, by design. If you experience any network glitch during play, including short wifi disruptions, the connection collapses. But it doesn't do so right away. It can take up to a few minutes for the game to notice that a player has disconnected. There is no automatic re-connect, all players have to save, exit, and rejoin. While playing Wesnoth it's advisable to stay in contact over email to coordinate.
Sometimes, the game state gets out of sync between players and the server. When the game notices that, you get the option to cancel the game or ignore and continue. If you do the latter, in-game causality breaks down completely (with hilarious results). If you opt for the former, once again, everybody has to save, exit, and re-join, and hope anyone has a non-corrupt save game. The game state isn't large or complex by the way.
These are not merely bugs; they are deep, architectural problems.