Hi,
I have read your exact requirement in which you have mentioned that you want your 10 existing OpenCart site should be more speedup.
Please share the URL of all sites so I can check the current performance and issues on gtmetrix and Google Page speed.
I will optimize the complete site and server so the load time wil be reduce and will achieve a very good score for mobile and desktop both.
Query: Do you have shared Hosting or Dedicated server?
I have +7 years experience in OpenCart and also have +5 years of experience as a Linux Administrator.
Following some steps so we can increase the speed of site:
1. compression
2. caching (in caching ...pagespeed ,varnish ,redis if root detail available)
3. leverage browser caching
4. optimize images minify css and js
5. add expires headers
6. database optimization
7. Setup CDN, etc
Regards
Karuna