Starting longer running tests sooner only make sense if one runs them in parallel with -jn.  I just tried installed 64 bit 3.6.1 with -r -j12 (forgot -ugui).  Total duration: 108 seconds.  The last to finish was test_multiprocessing_spawn after 84 seconds, 5 seconds after the 2nd last.  So it started at 24 seconds, when about 150 of 405 tests had finished.  To not be a blocker, it needed to start at about 19 seconds, when 120 tests had finished.

I re-ran with 32 bit repository debug build and was not so lucky.  Test 404 finished at 3:24.  Test_multiprocessing_spawn finished 1:33 later at 4:57, after 121 seconds.  It started at 2:57, when about 352 of 405 had finished.  Test 303 (3/4the) finished at 2:41, 16 seconds earlier.

Conclusion: the 3/4 restriction will help, but is insufficient to really balance process loads.