Google and other search engines have speed as a ranking factor. They think mobile sites should load below 1 second.
Different speed, mobile and desktop
Is there a good method to load the site with different speed? I can check with PHP what device that are used but nowdays sites are responsive with CSS.
Mobile site with speed
What is the "correct" way to make the mobile site much faster than the desktop version? How do I load the images differently when the site is responsive via CSS?
You can take a look on Yslow on website performance best practices and rules..
(taken from Yslow):
If you follow these already and still get a load time > 1s (you can check using this), you can look further on Responsive Images, there is a good read on that here, which basically means depending on the width of the browser you serve different sizes of images, which makes sense because for example why would you serve a '3000px' image on a '320px' mobile browser?
How, you say? One simple example is by using User Agent strings to determine if it is from a mobile/tablet device.. and serve the appropriate size of images depending on it. Take note however that UA strings are not 100% accurate and can be spoofed.
** If you happen to be using PHP, there is this library called mobiledetect that might help you..
** If you want to detect if the user is using 3G/2G, this SO question might help..