A typical third rail metro moves at around 25-30mph if you factor in station stops. Top speed varies, e.g. for Washington Metro the cars can accelerate up to 75mph but could plausibly be much slower depending on the track characteristics, how well sections of it are maintained, and how well the trains are scheduled. Hence it's unsurprising that Metro trains will get passed on a freeway unless there's a traffic jam.

Where Metro wins is on capacity. When the trains are full, one track of Metro carries an amount of people that would fill up 10 lanes of freeway. Of course, one major problem is that building mass transit is sold to the public as a way to 'reduce congestion' i.e. "other people will ride the subway so that my freeway will be clear". Due to induced demand from other people shifting their travel to the highway, this is not really truthful advertising.

It's more accurate to say that the mass transit provides an alternative for people who are fed up with traffic jams and want a more predictable way of getting downtown. Buses fail to provide that alternative, because unless the transit agency manages to get a dedicated bus lane along the whole route (it's probably not), they are going to be stuck in the same traffic that an ordinary car is stuck in.