Mace can see through the shrould of the darkside, maybe thats why he was able to defeat sidious, or at leat one of the reasons.
I dont think he froze them because mace was standing there and fighting...Others say Sids is truly better than all 4 and that he could have beat mace if he used the force, well dident yoda say that when him and dooku locked sabers it was a contest of the force? Yet Mace owned sidious when they locked sabers.... what does that mean?