If I open Google Chrome and Mozilla Firefox, with a few tabs active in each on popular sites, the entirety of both cores of my Intel E7500 CPU will be consumed by Flash advertisements.
I'm on a Linux machine with a lot of memory, which makes for the worst case scenario: First, Flash is horrible on Linux. Second, I use virtual desktops and leave browsers open for days at a time. Memory is not a problem.
Flash ads tend to be poorly written by a creative designer who could give a rat's rear end about your system resources.
The ads interfere with my ability to work, which costs me money. They also cause my computer to consume significantly more power. So in effect, your Flash ads are even bad for the environment.
They're also of course quite annoying, and if given only the options of browsing the internet with Flash ads or not browsing the internet at all, I'll choose the latter.
How about you try this experiment: Turn off Flash ads. Post a banner at the top of your site that says, "Hey, we've turned off Flash ads. Please exclude this site from your ad blocker so we can make money."