Does compatibility view in Explorer 10 slow down loading of pages?

Iam just curious because I have enabled compatibily view for all websites.

 

No.

If you have

it has other causes.

 

I am curious as to why you enabled compatibily view for all websites?

And, have you ever just tried adding trouble sites to the compatibility list, individually?

And, it could cause a variation in page (site) load times, depending on the pages. But not really anything you should notice, unless one or more errors are encountered.
Some pages (sites) are optimized for newer browsers and others are not.
All depends on how it was written.
If you read some info, in the links I posted below, I may give you more of an understanding of what I am trying to get across.

You can test the page loading times by using the F12 key in Internet Explorer and clicking on the emulation button and select your choice of emulation.
Then click on the Network button, press the start record and see the page loading results.
Don't forget to clear the cache after each test... it is one of the buttons in the menu at the top of the network page.

If you noticed, when you run the tests, you will see it running different scripts with the different emulations.

See links below for more info:
Using the F12 developer tools
Analyzing your webpage's network traffic
Emulate browsers, screen sizes, and GPS locations
Improving UI responsiveness