Using HD for the first time and the timeline preview is very slow - is this normal?

Yes, this is a known trade-off with the HD slide size. The spacebar timeline preview renders the slide live at full resolution, so HD slides with heavy images or video can stutter and show a spinner where the standard size felt instant. The good news: it's authoring-only. Your published course won't do this.

A few things I'd check: compress any oversized images (Storyline doesn't downscale the source files), and trim video backgrounds if you're using them — those two are the usual culprits.

And if you don't truly need HD, you can switch back in Design > Story Size. Just spot-check your layouts afterward, since objects can shift when the stage size changes.