Hi,
I have read your exact requirement in which you have mentioned that you want your existing opencart site should be more speedup.
Please share the URL of the site so I can check the current performance on gtmetrix, pingdom, etc.
I will optimize the complete site and server so the load time will be reduce and will achieve a very good score for mobile and desktop both.
Query: Do you have shared Hosting or Dedicated server.
Thanks
karuna