Why Ais Internet Connection So Slow On Iphone 4S?

I am using the AIS network on my iphone 4s. I noticed that the network signal is usually on Full Strength, but if there is the word "3G" next to the signal indicator, my connection to the internet would be VERY slow.

However if there is the word "E" next to the signal indicator, my internet connection would be reasonably OK.

I am very annoyed by the slow connection whenever its on "3G", but I dont know why sometimes there's "3G" and sometimes there's "E".

Does Dtac or True have the same problem? I was expecting that AiS has the best network so i am very disappointed with the slow internet connection.