To tell you the truth I have the same impression concernig MorphOS 1.4.5 on my A4000T with CSPPC. It's much faster comparing to 3.9
I cannot say anything about OS4 as I don't have it. I haven't even seen it live in action. Therefore I cannot say anything about it.
But it would be very nice to run MorphOS 2.0 on Classic Amigas even without all these eycandy features, but with new kernel, HDConfing and built-in TCP/IP stack.
