Even after doing all of this and using default optimization technique if you are not able to get expected load time. You need to check either you server is capable to run a system like magento or not. There are other things like full page cache and server optimization can be done.
We have worked on several magento websites for speed optimization and produced some unbelievable results. Techniques we use for magento store optimization is given below:
We review your css, js and html to follow latest web standards. We will minify css, js and html to give you best results and load time.
Optimize server for better performance
We setup expires and finetune server configurations to give you best load time. Its critical to have a well optimized server.
Configuring CDN to load css, js and images from another faster server
If your server is not able to scale or load static content properly. You can always choose a cdn provider. We will configure the same within few hours.
Configure full page cache and Varnish
Caching is important for load time and performance. Magento comes with cache. If its not serving the purpose you may look forward to implement full page cache or varnish.
Checking for possible malpractice and wrong query etc
We review sql queries being fired on product pages and other pages. We resolve all coding flaws and malpractices to give you well optimized website.
We also use other techniques depending upon the situation like images optimization. If you also fee that store should faster than what it is let a certified magento developer fix the issue for you.
If you want to an expert to install and configure your extension