I can try and help with 2, but not with 1, (never got the hang of seige assaults on mtw)

If the game is slow and choppy you have probably got the graphics set too high. Go into the options section and go to graphics and set all indicators to the low section, also change unit size too small. This should make the battles run ok. If they run fine with all setings on low you should be able to start altering them upwards until you get to a balance between graphics/unit size and game speed. If the battles are still choppy maybe you do need a new graphics card, but I think if you have a new pc you should be fine.