Currently, reference previews only appear after quite some time (350 ms) of hovering. While for #page-previews the delay works well, the much smaller #reference_previews (that often appear right after each other) benefit from a shorter time span. Especially, since reference targets are much smaller than page preview targets, and therefore less prone to accidental hovers, it makes sense to render the preview almost right away.
See also comment in T214693#4924357
 The delay for the appearance of reference previews is reduced to 150 ms.
 The delay for the appearance of page previews stays at its current rate.
- The delay is about waiting until the popup is being shown. Thus it does not matter whether you have been before.
- This ticket is independent from the bug that sometimes no popup appears if you hover too quickly between preview types
- The code is built in a way that we are never seeing empty popups, in the worst case the popup just takes a bit longer too appear
- The ticket is not about the popup to disappear faster, but about the popup appearing faster