Mobile Checkout Lag: Why You Are Losing 20% of Sales on iOS

By Valkyrie Built | Updated: January 23, 2026

Your desktop site loads in 0.8 seconds. Your speed score is 90. But your conversion rate on iPhone is plummeting.

There is a silent killer in Shopify themes that most developers miss: Input Delay.

When a customer on an iPhone clicks "Add to Cart" or "Checkout," nothing happens for 2-3 seconds. They tap it again. Still nothing. They think your site is broken, and they bounce. You didn't lose a sale because of your product; you lost it because your JavaScript main thread was clogged.

The "Main Thread" Traffic Jam

iOS devices (even the new iPhone 16) are sensitive to heavy JavaScript execution.

As we discussed in our Ultimate Speed Guide, when you install 20 apps, they all fight for CPU resources. When a user taps a button, the browser is too busy running tracking scripts (Klaviyo, Facebook, TikTok, Hotjar) to acknowledge the click.

This creates a "frozen" experience specifically during the high-intent moment of purchase.

The Fix: Defer and Delegate

You do not need to delete your marketing tools, but you must stop them from loading immediately.

We solve this by strictly managing Third-Party Execution. We ensure that Facebook Pixels and Heatmaps only load after the user's browser is idle.

By freeing up the main thread, the "Add to Cart" button becomes responsive again. No more rage clicks. No more lost sales.

Is your checkout losing you money on mobile? Let's audit your Input Delay. →