Paperless payroll management
Site speed can have several bottlenecks and seems like you only concentrated on the front end issues. The backend code and the database can be (and often are) site speed bottlenecks as well, especially if your issue is with TTFB.
Do you have NewRelic installed?
The things you tried are basic to do when you need to enhance your site performance. If the site is still slow, it may be because of themes or plugins. Also hosting has a very important role in the performance of your site. A shared hosting is good for small scale website, but it can still have an impact on your site performance because there are multiple websites hosted on a single server.
If you want a performance oriented server, you should go for a Managed Magento Hosting. It manages all your server technicalities and enhances the site's speed, making it performance and reliability better.
I would agree with @toniagency82ed that you should also look into backend and database factors too.
You also need to keep your Magento 2 app updated with the new updates like the new PHP versions have better performance than previous ones. If other packages are also updated to new versions then overall performance may improve.
We used to use Ceph clusters but they're expensive as you need 7-10 of them to make it work properly and that was expensive, but they did provide the performance, we've now found a better way using ARM processes but the install process is diabolical.
Seriously you will not get it running because virtually nothing is built for it and you have to install manually, but we have both Magento 2.x running on images and also Magento 1.x running on a different images, are they fast, you truly have no idea, Google goes nuts indexing and ranking although we do have 10,000s products which helps.
1. You need to find out what's slow: TTFB (time to first byte) or everything else
2. If TTFB is a problem - use a PHP profiler to find bottlenecks, try tideways or others
3. Run a pagespeed or lighthouse report and paste results