Nearly every slow site we audit is slow for the same four reasons, in the same order.
1. Images nobody resized
A 4000-pixel photo scaled down in the browser still downloads at full size. Resizing the twenty largest images usually halves the page weight on its own.
2. Too many plugins doing one job each
Each one loads its own stylesheet and script on every page, including pages that never use it. Audit what is installed and remove what is not earning its place.
3. No caching
Without a cache the server rebuilds the whole page for every visitor. Turning on page caching is a settings change, not a project.
4. Hosting that is cheap for a reason
Shared hosting packed with hundreds of accounts will cap your speed no matter what you fix above it. This is the only item on the list that costs money.
Work down the list in order. Most sites stop being a problem before item four.