Internet connection very slow only in my house and only with my Mac
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.