... Since websites seem to know when I have an adblocker running why don't they just intentionally slow the load time down.

Some do, even using prominent count-down timer displays in a few cases. Other sites quickly kill or fade out the page image if you're using an ad blocker. Still others overlay the sought-after page with a full-size notice that you have to turn off your ad-blocker to view the page. The techniques are endless...

However, depending on how the ad-blocking is accomplished, it can indeed slow down a page load depending on what the browser then does with the graphics space normally occupied by the ad.