When you are connected to your mobile network, several icons can appear (listed below by order of conection speed):

• E : (EDGE or Enhanced Data rates for GSM Evolution) is the slowest data connection. 

• 3G : (UMTS or Universal Mobile Telecommunications System) represents the 3rd generation data connection.

• H : (HSPA or High Speed Downlink Packet Access) is faster than 3G and is also called 3G+.

• 4G : (LTE or Long Term Evolution) is the last network mobile evolution and is also the fastest.

The 4G Network lets you have the fastest rates and provides a fast connection while the E network is the slowest connection.
However, true network quality depends on the congestion and the coverage.  This is the reason why it is possible to encounter a slow connection even when on a 4G network. 

Please note that you need to have a phone compatible with these standards to have them on your device.