Try this experiment. Clear a big space on the floor, out of the way. I used 6x10. Pile up next to it enough rocks to build the bridge, and enough blocks to build the bridge. Make sure that you have only one dwarf with architecture and masonry labors enabled, and give that dwarf nothing else to do. Shut off all other architecture and mason jobs so that your mason has nothing to do. Then exit-save and make a copy of the whole game folder.
Now run both games, side by side if you like. In the first game build a rough rock bridge in that space, carefully using only the rocks that are next to the bridge. In the second game, build the same bridge but out of the blocks that you've piled up.
Now let both games run. See who finishes the bridge first. If you want to be extra-scientific, note the dates of various events, such as when construction actually starts, when the bridge goes to the first stage of post, when it goes to the second stage of posts, and when it finishes. Also note any breaks that your mason takes along the way.
My data:
Oddommanang is a Legendary Mason +1, Proficient Building Designer. He's flimsy and very weak. He's building a 6x10 drawbridge.
Block Bridge
15 Felsite Begin work
19 Felsite Bridge goes to first construction stage of 6 posts.
20 Felsite Bridge goes to second construction stage of 15 posts.
22 Felsite Bridge complete .
Rough Rock Bridge
15 Felsite Begin work.
25 Felsite Bridge goes to six posts.
1 Hematite Bridge goes to fifteen posts.
6 Hematite Bridge complete.
I ran both scenarios twice, and these are actually the better data for the rough rock bridge. The other time, the mason loaded up all the stones into the bridge and then took a three-week break. I've read that stones are much heavier than blocks, so it makes sense that picking up 16 stones will make a dwarf tired faster than lifting 16 blocks. But breaks have some random component to them, so I don't have enough data to prove that lifting heavier objects will make him tire faster and therefore take a break sooner. It would make sense, but I can't prove it.
The summary is that the block bridge took 7 days to complete, while the rough rock bridge took at least 21 days (and longer when the mason took a break). Blocks aren't just lighter and easier to store; they cut bridge construction time by two thirds. And they probably cut other construction time by a similar amount.
I encourage you all to test this in other situations, maybe with a line of workshops or constructions. For now I'll assume that in all situations where you have a choice between rock or block, the block builds three times faster.
Edit: Updated the wiki article on blocks.