[Google Pixel XL Questions & Answers] Alcolawl:
That's pretty low. I get 350 mbps down and like 30 Up at home and my wireless download speed beats that. This is using an ASUS RT1900P Router connected to a Motorola SB6190 Gigabit Modem. There are many things that can come into play here.

First of all, let's understand what the Pixel is capable of. According to Qualcomm, the Snapdragon 820/821 is capable of 802.11ad and 802.11ac 2x2 MU-MIMO. This means a peak speed of 867 mbps. Pretty close to your gigabit connection's speed and more than you'll probably ever need for your cellphone. I must mention, however, that there is a thread around here somewhere that seems to have confirmed that wifi speeds throttle when the phone is not plugged into a charger. I'm not sure if that has been fixed but Google is aware of it. I have a feeling that this is by design.

You seem to have the right, if not the best, router for the job. Just try to keep in mind there's a lot of sorcery that goes on behind the scenes with routers and there might be options, depending on how savvy you are, that you can enable or disable to make things faster. QoS comes to mind, which I'm sure the router has enabled by default. If there are other people in your home or on your network streaming or downloading/uploading things the router is going to prioritize traffic a certain way so everyone's experience stays the same. Not to mention this will take a chunk out of the bandwidth available from your ISP. Additionally, the 802.11ad sounds awesome, but cannot get through walls that well. Like at all. It's purely Line of Sight. So unless you're in the same room as your Router, you're not going to experience the benefit of the Wireless-AD 60GHz madness. Channel Width has a direct effect on the theoretical speeds you can achieve with your wireless network. I have a feeling the router automatically chooses channel width based on the other networks in the area. Most of the time, the 160 MHz or 80 MHz channel width won't be selected by default, which seems like what you want. Read up on Channel Widths and why it isn't always beneficial to choose the widest one. The wider the channel width, the more channels you utilize, which means an increased chance for a nearby network to be using the same channel.

It's a good idea to micromanage which devices on your network are connected to which band. Try to keep slower, legacy and non-essential devices on the 2.4 GHz band. This preserves the speed of the 5 GHz band. In my home, I've passworded the 5 GHz network differently than the 2.4 GHz network and made sure to only add smart TVs, Streaming devices and phones to it. Other slower devices like older laptops, thermostats, and devices out of, what I would call, acceptable signal strength range of the 5 GHz network etc. are connected to the 2.4 GHz band.

I didn't originally intend for this post to be so long-winded but here we are. Basically, it could be any number of things. Try connecting your phone to the charger, connect to your 5GHz network, and run the test. Make sure you're running the test to the same server that you used when you ran the test on your hardlined computer. This way the numbers are directly comparable. If you live in a heavily populated area like an apartment building, there can be a fair amount of interference that will affect your experience. Luckily that band isn't as crowded and there are many channels that the router or you can choose from but, again, the wider the channel width the more likely you are to have interference. Speaking of playing with router settings, you can try optimizing your experience with things like channel width, channel, beacon interval, and QoS settings to start. Perform these tests at night when most people are sleeping to give you the fastest results.

Hope this helps. Let me know how it goes!

Source: The Internet, Netgear, Qualcomm, Cisco, Intel, my B.S. in Computer Security