I don't know about processor speed, but when comparing the 6230 to the older Series 40 phones you have to take into account that the 6230 has two times more heap memory available. If your application is pushing the memory limits on older Series 40 phones (which isn't saying much), then the garbage collecting might make the application run a lot slower.
There are performance differences among the Nokia devices due to the differences in the underlying hardware platform, which are not always logical, i.e. so that newer device would always have at least as much processing power as the previous devices. This can be for example due to different customer segmentation or different price point. This comes clearly visible when 6230, first Series 40 Developer Platform 2.0 device, has better performance than 5140. Therefore, 5140 is in fact better as the performance benchmark for other Series 40 Developer Platform 2.0 devices than 6230. As far as I know, all the coming Series 40 Developer Platform 2.0 compliant devices should have at least as good performance as 5140.
Well we have both a 6230 and a 6600 here. I'll bet they have the same or an equivalent CPU. The 6230 runs at least at fast when processing data as the 6600. For thos who don't know the 6230 has a mp3 player that can play 320kbits mp3, and therefore has a decent CPU.