Rijndael Speed Update
The original test configuration was using a 3meg file and the ECB cipher mode, which is basically a raw encryption on the individual blocks. Original scores were .NET @ 147ms and VBCorLib @ 302ms. After the massive optimizations, I brought VBCorLib to 172ms. That is a tremendous increase in performance.
I further tried an optimized 256bit block size version. The results were .NET @ 183ms and VBCorLib @ 183ms. A dead heat. Even using the heaviest cipher mode of CFB, VBCorLib is only 50ms behind on a 3meg file at 435ms to 385ms for .NET.
So with enough tweaking, I got more than what I was originally asking for. VB6 seems to still have some kick left.