Default [Q] Slow internet with Nexus 5 - Anyone using N5 with Straighttalk in SF?

I just received the new Nexus 5 (dec 17). I am using it with StraightTalk (AT&T network).
Since I activated the phone, I am experiencing slow internet and connectivity issues.
The network indicator on my phone indicates 'LTE' so I know I am connected on the LTE network.
I used the Speedtest app to determine my internet speed. Here is the kind of speed I have (in download):
With 1 bar signal strength: < 1 Mbs
With 2 bar signal strength: 1 to 3 Mbs
With 3 bar signal strength: 3 to 5 Mbs
With 4 bar signal strength: 7 to 15 Mbs
In average, I have no more than 2 to 5 Mbs in San Francisco. Strangely my upload speed seems almost normal, and is always higher than my download speed (as high as 5 times higher), which is weird.

It seems rather slow to me for a LTE (4G) connection.
People are reporting speed as high as 25 Mbs with the Nexus 5 (even on StraightTalk), and 15 Mbs with only 2 bars of signal.
I know many things could explain a slow internet on a mobile phone: your local LTE network, the setting of your account by your network provider, your network setting in the phone (APN), a defective phone, etc.

In order to eliminate most of these possibilities, I would like to know if anyone is using the Nexus 5 with StraightTalk in the San Francisco area and is having higher average speed than what I have?
If it's the case, I would have to conclude that my phone is faulty.


For information, my APN setting is the following (recommended by StraightTalk Costumer Support):
NAME Straight Talk
APN tfdata
Port 80
MCC 310
MNC 410