iPhone WiFi Speed Issue iOS 8.1

Hello,

I just bought a 32GBs Iphone 5S phone which has IOS 8.0 OS running. i had some Wi-Fi internet issues which made me to update it to IOS 8.1 which said to be have a better Wi-Fi speed. After i updated, the Wi-Fi speed got better in some programs, but i still have Wi-Fi speed problems for messenger programs ( like whats app , line and ... ) and also some online games like Clash of Clans. I've Just tested my Wi-Fi speed on my friend's phone ( with the O.S of Android ) which had no simillar problem and it was just O.K. I think the problem is from iPhones with iOS 8 up to 8.1.