What may be in play here, is what Cletus is referring to. If both of you are actually at whichever version started the pre-load of preceding and following images.. It may be that you notice the "loading" display if you move through the nearby images 'too quickly', before LR has had a chance to build the nearby previews.
(ETA: Come to think of it, I think they added that a while ago - but then 'tuned' it in a later release or 2.)
However, having said that, I've been noticing similar when zoomed in at 1:1 and going back and forth between 2 next-to-each-other images to compare them (in Develop). It may be fine for 2 to 3 switches or so, but then suddenly it'll say "loading" on a subsequent switch - and then it repeats after a varying number of more switches. It causes a hefty problem when I'm trying to compare noise in a bracketed HDR set - between the LR-merged 2-exposure and 3-exposure versions. Though I also notice it even when zoomed 1:1 into non-merged raws, albeit it goes quicker.