Internet connection very slow only in my house and only with my Mac I bought a brand new MacBook Pro 13" 2015 (8GB RAM, i5, 128 SSD) 7 months ago. The thing is, my Mac works and I love it, but a couple of months ago, without doing anything, loading web pages even with Chrome as with Safari is extremely slow. My girlfriend has a Windows laptop and it works perfectly and when I connect to a different Wi-Fi router it works great. So my guess is there is a problem with my Mac related to my router.

I have tried several solutions already - I changed all of the DNS servers you can imagine in macOS and on my router as well, restarted my network settings, excellent namebench to see which is the right DNS for me, clearing caches... But nothing helped.

I must say that I don't have this problem 100% of the time, but more like 70% of the time. Sometimes it's like suddenly the problem goes away.
Speed test gives me 52.42 mbps download and 3.09 mbps upload and I always connect via Wi-Fi and not ethernet.

Follow-up:

This is the result of the following command: 

ping -c 100 -s 1992 -q 192.168.1.1
--- 192.168.1.1 ping statistics ---  
100 packets transmitted, 20 packets received, 80.0% packet loss  
round-trip min/avg/max/stddev = 2.424/10.420/56.773/11.580 ms


80% packet loss sound like a lot. What should I exactly do with iStumbler?
Internet or Wi-Fi problem

Open System Preferences... &gt; Network &gt; Wi-Fi &gt; Advanced... &gt; TCP/IP
and note the IP address of your Wi-Fi router.

Let's say it is 192.168.1.1.

When you are in your bad network situation,
open a Terminal and enter the following command:

ping -c 100 -s 1992 -q 192.168.1.1


to check your Wi-Fi quality of connection and let run this command for nearly 2 minutes.
Make a copy of the result and include it in your question.

If is tells 0% packet lost, then your problem is a general Internet problem. If it tells your that you have some packets lost, then your
problem is on the Wi-Fi side.

I suspect that this might be your case.

Analyse a Wi-Fi problem

Download the iStumbler software:

iStumbler

This version is free:

iStumbler 99

This is an excellent quality software and running without problem on from Snow Leopard to Yosemite (I didn't yet tested it on El Capitan or Sierra).

When you are in your bad network situation,
let iStumbler run for 10 minutes, order the output along the Level
column in decreasing order. Make a screen capture and include it in your question.

IP or DNS problem

Open a Terminal and enter the following command:

dig www.apple.com


This command will ask your DNS server how it does resolve the www.apple.com in an IP address.

Include the output in your question.