PS2 emus are just as slow as Gamecube emus. Xbox "emus" are really just clever wrappers so they can be fast.
PowerPC emulators are slow, thus Dolphin is slow. The 64-Bit Gekko is arguably as fast as the XBox's Intel Pentium III/Celeron hybrid @ 733MHz, so emulating a CPU this fast is going to give single digit frame rates on any computer of today. On top of this, you need a DirectX 9.0 card to even hope to display graphics correctly.
Also bare in mind that the N64's 64-Bit CPU @ 93.75MHz takes 1 GHz or so to run smoothly on a well-developed COMPATABLE emulator, and the Gekko is about five times the speed. Emulation NEEDS a decent CPU, and my 3200+ Athlon is well below minimum requirements for the 3D games.