I was just noticing the fact that for most sites it seems the speedIndex is roughly the same for both repeat view and first view. Is that logical/theoretically expected?
Not necessarily, it depends on how well they do at caching content. This huffington post page differs by ~30% for example: http://www.webpagetest.org/result/140801_RV_64D/?medianMetric=SpeedIndex
They will usually be relatively close (not an order of magnitude faster) as long as the base HTML still needs to be delivered and if they are reasonably well optimized.