Short answer: not if it is built for a shop. Lighthouse on Dawn and on Horizon shows no drop with Segue switched on. The longer answer is more useful, because it tells you what to look for in any transition, ours or not.
What a transition adds
Two things: a script, and work on the click. The script is the part Lighthouse sees. Segue sends only the chosen style’s code, and the click script is under 9 KB; nothing runs at load except registering for clicks. The work on the click happens after the visitor has decided to leave the page, which is the one moment a shop is not being measured.
What a transition must not add is work on every page: a canvas that draws constantly, filters over the whole page, listeners on scroll. Those show up in Total Blocking Time and in the feel of the page, and they have nothing to do with the move itself.
What it hides
The blank. On a plain theme, the time between a click and the next page is white, and the next page then assembles in front of the visitor. With a transition, the cover closes on the click, the next page loads behind it, and it opens when the page is ready. The same milliseconds pass; they are spent looking at your colours instead of at nothing. Perceived speed goes up even though measured speed is unchanged.
Segue goes one step further: as the pointer reaches a link, the next page is already being fetched. By the time the cover has closed, most of the page is there.
What makes one slow
- Taking over the routing. Libraries that turn the theme into a single-page app have to re-run every script on every page, and fight the cart drawer, section rendering and every app that injects itself. That shows up as jank and as bugs, and bugs are the slowest thing of all.
- Fixed durations. A transition that plays for a set time and then shows whatever has loaded gives you either a cut-off animation or a page that is not ready. The cover should open when the page is, and not before.
- Heavy covers. Blur filters, large video, dozens of animated elements. A cover is on screen for well under a second; it should be cheap to draw.
- Running everywhere. Cart, checkout and account pages should never be covered. Segue leaves them alone.
How to check your own shop
Open PageSpeed Insights, test a product page with the embed on, then off, and compare Largest Contentful Paint and Total Blocking Time. Test on a phone profile; that is where most of your visitors are. Then click around the shop yourself on a slow connection, because the feel is what the numbers cannot show.
Where Segue stands
- Only the chosen style’s code is sent; the click script is under 9 KB.
- The next page is prefetched as the pointer reaches a link and loads while the cover is shut.
- Nothing in the theme’s code is edited. Switch it off, and the theme is exactly as it was.
- Lighthouse on Dawn and Horizon: no drop.