068 - Stop Blaming the Theme: What's Really Killing Your Shopify Store's Speed

Shopify performance specialist Izaac Barratt of Baseline Commerce joins Karl and Taylor for a deep dive into site speed optimization.

The conversation covers the full spectrum from LCP and INP fundamentals to the real-world friction of app bloat, AI coding habits, and what actually moves the needle for conversion rates. A must-listen for any theme developer, app developer, or agency working with high-traffic Shopify stores.

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Find Izaac Barratt Online

LinkedIn:

https://www.linkedin.com/in/izaac-barratt — where he posts performance tips, resources, and his ongoing series on the nine performance fix buckets

Baseline Commerce:

baseline-commerce.com — Izaac's site speed optimization consultancy

Timestamps

01:23 – Intros; welcoming guest Izaac Barrett

02:48 – Izaac's origin story: animation, Newgrounds, and e-commerce

05:36 – What drew Izaac to performance optimization

06:00 – First site speed project: 12 seconds to 2 seconds, conversion doubled

10:00 – LCP fundamentals: above-the-fold prioritization and perception

18:00 – Preloading, lazy loading, and how to get them wrong

20:43 – Liquid's preload tag, sections_index, and reducing JavaScript dependency

26:38 – Telling AI to prioritize HTML/Liquid and CSS over JavaScript

31:01 – Async vs. defer vs. render-blocking scripts explained

35:28 – Apps and performance: identifying problem apps, merchant conversations

39:15 – Advice for app developers on loading strategy and user interaction cycles

41:42 – INP (Interaction to Next Paint) explained: what it is and why it's hard to fix

43:30 – Shopify/research report: 34ms of INP = 3.5% drop in CVR

46:06 – How to debug INP: Chrome DevTools performance tab and CrUX data

53:20 – Izaac's LinkedIn series: nine buckets of performance fixes

56:17 – Dev Changelog

1:02:44 – Picks of the Week

Dev Changelog

- Next-Generation Events — Developer Preview: https://shopify.dev/changelog

- ⚠️ Expiring Offline Access Tokens Required for All Public Apps — January 1, 2027: https://shopify.dev/changelog

- Shop Minis — March/April 2026 Update: https://shopify.dev/changelog

- Customer Account UI — Feature Preview: https://shopify.dev/changelog

- App Home as a UI Extension: https://shopify.dev/changelog- llms.txt / agents.md in Shopify Themes: https://shopify.dev/changelog

Picks of the Week

Karl:

The Dark Journey: The Tragedy of the Donner Party

by Allan Eckert – A true history of the ill-fated 1846 wagon party, written by one of Karl's favorite frontier authors; dark but compelling, especially for history enthusiasts. https://www.amazon.com/Dark-Journey-Allan-W-Eckert/dp/1931672539

Izaac:

Page Speed Impact Estimator fastersite.ai – A tool for estimating the performance gains of removing specific scripts and assets before actually doing the work; great for benchmarking and scoping conversations with clients.

Taylor:

The Will of the Many

by James Islington – A sci-fi novel recommended by Taylor's brother (a bookseller); follows a teenager hiding his royal lineage in a Roman Empire-style society that literally harvests people's will to gain power. Book two also out. https://www.goodreads.com/book/show/58416951-the-will-of-the-many