Slow WiFi connections with MacBook Air and Asus RT-N14UHP

Hi, I am encountering a strange problem whereby my MBA is having slow wifi connection when I am connected to my new Asus RT-N14UHP.

Other devices that I have (i.e. Macbook Pro and smartphones) do not seem to have that problem. FYI, the router is located one floor below my room.

I tried connecting my MBA to my office's wifi network and there is no problem as well.

Other info that might or might not be related is that I just upgraded my MBA SSD card after the older one died. With the older version installed, there was no such issue (also maybe because I was connected to the old router).

Could it be caused by the new router or is it router+MBA combined?

Let me know if you need any specific info to figure out the root cause.

Thank you in advance