Checkout feels like the highest-leverage place to intervene — it's the last step before revenue. But a checkout extension can only improve conversion for people who actually reach checkout, and most funnels lose far more people earlier, on product pages and cart.
Product page load speed, clear shipping/return info before cart, and cart abandonment triggers typically have more total impact than anything checkout-side, because they affect a larger population of visitors earlier in the funnel.
Once the upstream funnel is solid, checkout extensions are genuinely valuable — post-purchase upsells, custom validation for B2B order minimums, localized payment options. Just don't let it be step one.