Zoom In on "Checkout Is Slow"
Work from the brief alone. Write what you would produce — actors, requirements, data, a first slice, your unknowns — before opening anything below it; the exercise stops working if you read the reality first.
Support reports checkout "feels slow". Starting from the user, descend one abstraction level at a time — user, request, service, dependency, query, storage — and at each level say what you would measure, what reading would send you down, and what reading would stop you. Do not skip a level without a measurement.
Your attempt
Nothing here is checked. It exists so that the reveal below is a comparison rather than a reading.
Opening the database profiler first because disk-latency graphs are available and dramatic. An afternoon is spent on a storage layer the request never waited on, a faster disk is ordered, and the analytics script on the confirmation page is still there.
Read this even if you are confident. It is here rather than behind a button because it is the move most people actually make, it produces things that look like a project, and its cost arrives when the first hard requirement has nowhere to go.