Slow Internet Problem On Yosemite

Internet connection is really so slow on MacBook Air 13' Yosemite . Called tech support of ISP and they said my connection is OK . My internet plan has 16 MBPS speed but when I do a speed test on my MacBook, it's just 2-3 MBPS . But in my mobile phone, internet connection is really good. So I thing I have a problem with Mac OS configuration.

These are my System Preferences > Network settings :

BTW , connection is not slow always, sometimes it's getting better. For example it's took about 10 minutes to upload these 4 images, but now my connection is better.

Can you tell what should i do for better connection ?

Note : I can't try it without Wi-Fi , with an ethernet cable, because I don't have a ethernet slot on my Mac.