Just for fun, I downloaded the 9 Beta on a Mini 1 (512 MB Ram and the A5 chip) a couple weeks ago. Obviously no multitasking, but also no issues. It seems to handle memory a bit better as I've had very few app crashes. I run MS Outlook, Safari, Kindle and Mail simultaneously and, with the exception of Safari still refreshing pages, haven't had apps just closing as they did in 8.
I have been able to run those four apps as well as FB, SalesForce and my banking app while not crashing. I don't do it often or for very long as everything lags, but it's impressive for such an old thing.
My Mini 4 is coming tomorrow!!