35% reduction to speed and damage when you use a two-hander in one hand. This includes horseback weaponry. Additionally, whenever you use a shield with a 1H/2H weapon, it uses 1H WPF instead of 2H WPF. 1H/2H weapons used without a shield on horseback use 2H WPF (unless you can switch the mode, like some one-handed bastard weapons; then it simply uses whatever mode you select).
Edit: However, if you use Vargas' web tool, it gives different numbers of reduction depending on the type of weapon, shield used, mounted, etc; the percentage I provided might not be 100% correct.