Replatforming is the most expensive possible response to a slow store, and it frequently does not help, the new platform inherits the same oversized images and the same twenty scripts. Fix the causes first; you can always replatform afterwards with better information.
Start with what is actually being downloaded
Open the network panel on a product page and sort by size. In most stores the top ten entries are images that were uploaded at print resolution and a handful of apps nobody remembers installing.
- Resize and re-encode product imagery; serve modern formats.
- Remove apps that are no longer used, each one usually ships its own script.
- Defer anything not needed to render the page: chat, reviews, trackers.
Then fix the queries
Collection pages are the usual offender: an unindexed filter or a per-product lookup inside a loop turns one request into hundreds. This is invisible until traffic arrives.
Then cache
Put a CDN in front of everything static, and cache the expensive, rarely-changing fragments. Caching last is deliberate, cache too early and you hide the real problem instead of fixing it.
Caching a slow page makes it a fast page that is still slow for the first visitor, and for every cache miss.