I think, there is an example in the book, where Harry puts up a ward in 3 exchanges, so 1 "pass" of summoning power is equal to 1 exchange.
I think the big difference is, that you need the entire exchange for your thaumaturgy ward, even if you pull it up in 1 exchange. That means, that anything that happens might break your concentration, and result in fallout/backlash and the spell not finished. Thaumaturgy at the speed of evocation is much more than thaumaturgy in 1 exchange, because it does not last longer than throwing a fireball or something, you just wave your hand, mumble a word and voosh, there is your ward, and there is no way to interrupt you like a thaumaturgy ritual (and even if it takes only 1 exchange, it still is a ritual).
At least that is how I have decided to use the rules, because as crusher_bob said, they are pretty vague.