I just got my M8 a couple of days ago.but am experiencing problems with the Wi-Fi download speeds. I've got a 30meg connection at home which is working perfectly fine (I have tested this by running speed tests from multiple devices, including my old HTC One X and my partner's Xperia Z) and they all do the test at maximum speed. The problem is that my new M8 gets results of between 3 and 5mbps on the same speed test (By Ookla). I have not changed any of the settings and don't have any power management apps or anything that's likely to do this.
If I disconnect and reconnect to my router, the speed test results get the expected download speeds for a short while, but then return to the slow ones.
Sent from my HTC One_M8 using xda app-developers app