Every percent of checkout conversion lifted shows up directly in revenue. We treat the checkout as a product of its own.
First, the bar to clear: one-page or accordion, mobile-first, with Apple Pay / Google Pay above the fold. If you're not doing this, start here.
Then the optimizations: trust signals (returns, shipping, secure-checkout badges), guest checkout always on, autocomplete on every address field, and post-purchase upsells for the recurring win.