Site speed should be a concern for websites. Google introduced site speed as a signal in its ranking algorithm in 2010.  Whether it's a new design adding burden to the server, a content management system (CMS) slowing page construction, or even third party JavaScript degrading response times, it can be difficult to achieve desired results in a timely fashion.  With devices using many different networks (with varying bandwidths) to access sites, it’s hard to achieve consistent performance without proper optimization. Making a site responsive to screen size is only the first step. Improving site speed often requires some sort of compromise and discovery. However, with a considered review of the site content, server resources, and software capabilities you should be able to get a good idea of where to look for areas of improvement.
 
Options for site optimization:
 
1. Server side caching - Retain pre-built content for delivery
2. Asynchronous Delivery - Provide data after the page load
3. Compression - Reduce bandwidth required to send data
4. Client side caching - Enlist the browsers to serve previous data.
5. Distributed delivery - Use third party servers to provide static content.
 
NOTE: Each of these may require multiple changes to improve page loads.
 
Here are some examples of how each may affect your site speed.
                                        
Server side caching
When a Content Management System (CMS) receives a request for a page it runs through thousands of lines of code, multiple calls to the database, and returns multiple files to help theme the page.  While these operations are relatively fast, they also utilize server resources. Even if a full page build is only a fraction of a second, it adds to the other steps in the page request process causing slower page loads.  
 
By caching the pages, we can skip the page build and database calls.  This allows the server to instantly return the requested content.  If the content only changes once a day, then you only need to create a new page once a day. However, until the cache is cleared, every request will result in the cached copy of the page.  
 
Asynchronous Delivery
This is a technique that loads the more intensive parts of a page after the primary content. You know asynchronous delivery is being employed when pages load with gaps - missing images, blocks of empty content, progress bars, or spinners. When properly planned, asynchronous delivery can allow content to load quickly while large images arrive later.
 
Compression
Typical web pages can be reduced by 50% or more in size by compressing the content.  The browser will then decompress the content before display on the screen.  The addition of compression and decompression does add to the time required to load a page, however, the speed of transmitting less data makes up for the effort.  This category of site performance also includes reducing the number of files required to load a page and minifying files. Compression can be one of the quickest and simplest gains in site speed, but it can have some unintended side effects, so it should be used with caution and in conjunction with thorough testing.
 
Client side caching
All website pages have an age and a max-age property.  These two items together allow a browser to hold a local copy of the content for an established period of time.  By using client-side caching, the browser can instantly display content that it has already been visited.  Often we find sites that do not provide an age or max age for their content. In turn, the browser must ask for the files over and over again regardless of how old the images, files, or content actually is.
 
Note: While this can increase site speed, managing this caching information is not simple because every browser handles this differently.  
 
Distributed delivery
Lastly we reach the holy grail of speed: making someone else “pay” for high availability web servers.  This category of optimization is tedious to setup and can be fairly expensive, but if your goal is fast delivery without the worry of server availability then this is the solution for you.  Content Delivery Networks exist in two flavors: small players who will host large files to reduce the load on servers, and large players who will provide multiple servers at various locations around the world.
 
In summary, there are often many simple solutions to site speed that can pay quick rewards. Some solutions need more effort and resources to realize the benefits. The first step is to evaluate where the slowdowns are occurring then prioritize where to spend your resources.