Why does my page jump while images load?
Because the browser does not know their size until the bytes arrive. Set width and height attributes, or an aspect-ratio in CSS, and the space is reserved from the first layout pass.
A six column table is unusable on mobile. Options?
Either let it scroll horizontally inside a container with a visible edge fade, or restructure into cards below a breakpoint. Hiding columns is the tempting third option and the one people regret, because the hidden column is always the one someone needed.
Is prefetching links worth it on a content site?
On hover, yes, and it is nearly free: by the time the click lands the document is often already there. Prefetching everything visible is the version that burns other people's data plan for a marginal gain.