It also depends on the size of your project and if you are re-building or just building.
With Qt even medium sized project on not very fast system can take some time to compile as the moc'ing also comes in.

When you rebuild, then all the temp fails, objects etc are deleted, and then everything needs to be compiled and built from the start again.
If you just changed something in a source file (not so with headers) then only that compilation unit will be compiled and linked to the already existing objects, which is much faster.

What tbscope mentioned is of course possible as well.