Given and init.sqf (or initIntro.sqf) with the following structure:
finishMissionInit;
startLoadingScreen ["..."];
// code that takes a while...
endLoadingScreen;
...then the mission will start anyways, given that code takes long enough. With an init.sqf and previewing the mission from the editor, you're simply dropped into the game at some point. Using initIntro.sqf, you're prematurely dropped to the map-screen in a similar fashion.
Attached is a simple sort-algorithm benchmark, programmed to test implementations of two shell- and a heap-sort, up to lists with 15000 elements. On my system the mission starts (prematurely) after the lists with 8000 elements have been sorted (step size is 1000). The mission then starts, while the lists are still being sorted; note/observe the diag_log output (i.e. the initialization script is still running).