Storefront Performance Investigation

A merchant’s storefront started failing on roughly half of all page loads, on every kind of page, with nothing in the theme settings to explain it. We wrote a small script that loaded the storefront fifty times and recorded what came back, how long it took, and a number Shopify quietly attaches to every page saying how much work that page cost it. The cause was a handful of lines in our own add-on that re-read the store’s saved settings on every single page, pushing each one past a cost limit Shopify does not publish anywhere. We published the entire investigation, including the four hypotheses that turned out to be wrong and the probe script itself, so any merchant can measure their own store.