Every extra second a Shopify storefront takes to load costs conversions, quietly and consistently. The frustrating part is that most stores are slowed down by a small handful of fixable issues, not some deep unsolvable problem.
Where the slowdown usually comes from
The most common culprits are oversized, unoptimized product images, too many third-party apps injecting their own scripts on every page, and themes that load far more CSS and JavaScript than the page actually needs. Individually these seem minor; stacked together they add up to seconds of load time.
What actually helps
Compressing and properly sizing images (serving the size the layout actually needs, not the original upload) is usually the single biggest win. Auditing installed apps and removing the ones that aren't earning their keep is the second. Beyond that, reducing render-blocking scripts and making sure the checkout flow itself is lean, since that's the page where speed most directly affects whether a sale completes.
Speed and checkout conversion are directly linked
A slow product page costs you some browsers. A slow checkout page costs you people who had already decided to buy. That page deserves the most attention, since it's the closest point to lost revenue rather than lost browsing.
How to think about a rebuild versus a fix
Most speed problems don't require rebuilding the whole storefront. They require a focused pass on images, apps, and the checkout flow. A full rebuild becomes worth considering when the theme itself is old, heavily customized in ways that are hard to maintain, or when the mobile experience specifically is the weak point, since that's where most Shopify traffic actually shops.