A Force user won't necessarily be blitzed by an opponent like Sidious just because they can't see them with their eyes, but, given that Force Users have superhuman perception speed, and usually have no issues with seeing each other when they fight, the fact that Sidious can move faster than some Force users can see clearly, or at all, suggests a large speed disparity.
Having said that, Force users have a degree or supernatural clairvoyance - the Force guides them, they can see through the Force like a large sphere of vision, they can see things before they happen - which is why despite the fact Sidious has moved uncomfortably fast for the perceptions of Maul, Windu, Fisto etc, they've all still managed to react to him and put up some level of a fight (Maul's can be partly attributed to his rage enhancement from Savage's death, which Fisto lacked, which is why Fisto basically got destroyed and Maul faired better.. because Maul isn't leagues ahead of Fisto usually). Windu sinking into Vaapad let him ramp his speed up to Sidious' level in the RotS novel, which explains why in that fight he went from being his usual self - someone who would have his hands full with Maul in a fight - to being the second person to date capable of matching Sidious in a fight. Then there's the business with Sidious throwing the fight, the false shatterpoint etc, but that's not really important. As for Kolar and Tiin, the RotS novel indicates that Sidious had a certain level of surprise on his side. People often overlook that, which is why they, and Fisto to a degree, are considered worthless by most people. In all reality they are very high calibre fighters, and are noted as such, they just got caught cold by the most powerful Sith in history, the same way plenty of fighters would have.
But yeah.. having said all of that, Operation Knightfall Vader with three days to prepare for Sidious - to focus himself, come to terms with his potential, learn to direct it in a way he hasn't done before - would do just fine against him. The guy is a complete monster who improves by the day, and with enough motivation can become substantially more powerful than he's ever been before. Like when he curbstomped Dooku with the right motivation. It's speculative as to whether or not he would win against Sidious, but he'd give him a hell of a fight.