Searched the bug reports page, didn't see this. Vanilla unmodded DF, with invaders off, wined on Linux (although I haven't gotten this the other times I've been wining it)
A few days ago, this fort's first dwarven merchants arrived. The first wagon started slowly moving over my smoothed stone "road", slower than they normally go. Like, really really slow.
Then, the second wagon arrived at the site, and pretty much warped to the depot. It moved as fast/faster than a flying bolt, and instantly started setting up. It took another like five minutes for the first wagon to arrive.
I sort of wasn't really thinking at the time (late at night) so I failed to do a few things, including checking what was at the depot before/after the slow wagon and the mules arrived, or getting a save while it was at the depot. When the merchants packed up and left, the fast wagon again warped off the map, although I don't recall if the slow wagon was still slow. I think it was just normal-speed after, though.
So, not such a useful bug report if it isn't something obvious in the code (I would expect weight-related- I bought a lot of metal and an anvil off the merchants, which I suspect were all on the slow wagon). If it happens again next time traders come, I'll get a save in the middle and post a link in this thread.