Speed is rarely one big problem. It is usually many small ones. Start with these five checks.
1. Image size
Upload images at the size they are shown, and let Shopify serve the right version for each screen.
2. Unused apps
Remove apps you no longer use, and check that they left no code behind in the theme.
3. Third-party scripts
Chat, tracking and review widgets add weight. Load them late, and only where they are needed.
4. Fonts
Use as few font files as you can, and keep text visible while they load.
5. Measure before and after
Test a few key pages on a phone connection before you change anything, then test again. Keep what helps and undo what does not.