Can you fix slow Magento checkout specifically?

Yes, checkout is one of the most common Full Optimization scopes. Typical wins: (1) defer all non-essential third-party scripts (chat, analytics, A/B test tools) until after the order is placed, (2) tune the shipping-method and tax-calculation API calls (parallelise where possible, cache where allowed), (3) trim the KnockoutJS view-models that ship with Luma checkout, (4) if you’re open to it, drop in Hyvä Checkout, one-page Alpine.js checkout that loads in < 1s and ships natively in 2.4.9. Checkout-only Lighthouse scores typically jump from 40-60 to 85-95 after this work.