The unpredictable loading bar -- or, the slot machine effect
A couple of studies (Harrison et al, 2007; Amer &amp; Johnson, 2016) tested pauses/stalls in progress, and concluded (respectively): &quot;... participants perceived progress bars with pauses as taking longer to complete ...&quot; and &quot;... participants preferred a progress bar that moves consistently to a stalling progress bar, and judged the process duration to be shorter ...&quot; suggesting that continuous/steady progress bars are preferred by users over intermittent/random indicators.
There is much more research on continuous progress indicators (eg, Villar, Callegaro, &amp; Yang, 2013; Liu &amp; Wronski, 2018) that suggests that users generally prefer fast-to-slow / decelerating progress - that is, inaccurate progress bars that overestimate progress early on.  Slot machines similarly use early success to increase player perseverance, a kind of anchoring bias.  However, most of this research is on progress indicators for active tasks such as filling out surveys.  Some experiments with passive waiting (like file transfers) align with their results (eg, Gronier &amp; Baudet, 2019; Kuroki &amp; Ishihara, 2015), but other studies show a preference for accelerating progress instead (eg, Harrison et al, 2007), so I would wait for a meta-analysis specific to passive tasks before making a firm conclusion.
There are also various exploratory studies testing alternative progress bar displays, such as different colours and shapes, designs and animations, adding audio and video, suggesting that user perceptions can be altered in a variety of novel ways.  As well, progress indicators on passive tasks may be affected by the labour illusion, in which user experience is influenced by the perceived amount of work done.