I installed this module a couple of days ago and used it up until now. When I noticed that my site is running VERY slow. I tried to find out what was causing it and I found out it was Random image extended. I don't know why but when I used a random image module on my old 1.5 website with the same images it wasn't running slow so it can't be the images fault. Still... It's a great clean looking module and I really like it. If it wasn't for that one fact...
RIE is a well written and optimized on speed module. In my tests with a large amount of big images (over 100) the whole execution took only some hundred milliseconds, so it is not true what you say. Maybe you need to change the server or the hosting company where your site is hosted?
By the way: why don't you ask for support first instead of writing reviews with little truth in it?