The issue may occur due to your site's backend performance from your WordPress hosting provider. As if your hosting is slow, there is the highest possibility that your website will be slow even if you have done a lot from the front-end optimization.
Use a Content Delivery Network (CDN) for faster and quicker site loading.
Optimize your image by resizing the image, and then you have to compress it to reduce; further its file size.
Optimize CSS by Minifying CSS code and reducing file size
Also, minify and combine JavaScript; however, exclude specific javaScript files to avoid any issue.
Offload media files to external storage such as Amazon S3, DigitalOcean Spaces, Google Cloud Storage, etc.
Limit or disable revisions to prevent a clutter of Wordpress stores that store unlimited revisions whenever you hit the button Save draft.
Regularly cleaning your site database as a more complicated database makes your site performance down; the reason is that when someone visits your site, your server queries the database to get information to render the page. So frequently, it makes your database more complicated.