timdiggerm said:

As much as I usually hate overexplaining fiction...

With sublight engines pushing for long enough, the Falcon achieves near c flight. A week passes on board the Falcon, while Luke trains with Yoda for months. Boba follows long enough to figure out their course, and can easily predict when they'll arrive. The Empire, thus, arrives just before the Falcon.

That doesn't really work either. It would still take years from everyone else's perspective to get from one solar system to another, even traveling at near-light speeds. And Darth Vader is not depicted in this movie as being a particularly patient dude. If it took the Falcon that long to get to Cloud City, there is no reason why it could not have been intercepted by the Empire, who could easily get the drop on any ship traveling at sublight speeds.

Of course, as hairy_hen pointed out, the actual reason why this was able to happen was because it was written in the script.

However, my own “explanation” for this plot hole is that the Avenger made a quick hyperspace jump to rendezvous with the Executor and rest of the fleet, which has clearly gathered again after having been scattered at Hoth and the Falcon pursuit, before Vader orders it deployed.

This is consistent not only with Threepio's attitude when we cut back to the Millenium Falcon's cockpit — it would have been quite shocking to have a sudden hyperspace jump into the middle of the Imperial Fleet — but also because the sequence in which the fleet is depicted breaking up does not feature the asteroid belt “horizon line.” This was a constant visual reference throughout the film up until this point, but the shots of the Imperial Fleet, the Falcon detaching, and the pursuit of Slave One feature a pretty wide field of view, during which the asteroid belt is nowhere to be found, implying that they're somewhere else.

Also, why would Han have to bother to check his position if they never left the Hoth system?