Treat checkout as the last step and you'll tune everything ahead of it while conversion sits still. Treat checkout as the product and your priorities reshuffle in a hurry.
Speed is a feature nobody sees
Product-page load time is the most overlooked conversion killer we run into. Every image, script, and third-party app has a price, and on a mid-range phone over 4G those prices pile up into whole seconds. Set a speed budget and make every asset earn its place against it.
Ask for less, later
A checkout that makes people create an account before it will show a shipping cost is asking for commitment you haven't earned yet. Default to guest checkout, show the full cost and delivery date early, and drop every field that isn't strictly needed to place the order.
Trust the numbers
Half of this work is being able to trust your own analytics. Clean, consistent tracking across the funnel is what turns 'it feels slow' into 'shoppers bail at step three on mobile,' and only the second one gives you something to act on.