While mobile devices are replacing PCs for almost all possible applications, the Internet is also changing to address this technology shift. On the other hand, despite the unbelievable progress in the performance characteristics of smartphones and handheld computers, they are doomed to stay different from PCs by one parameter: screen size.
That is why most web sites offer different content when they are visited from a mobile device.
Of course, this content is usually just a reduced version with a very similar functionality. However the implementation of the underlying web application can be different.
As a result, your web site can experience significantly different performance problems when it is accessed by multiple mobile users.