Ports sometimes suffer from slowdown.
For instance Ikaruga for DC has slowdown that the Naomi version does not.
Other way round. The DC rev slows down a little bit less than the NAOMI original does. This was confirmed early on by both Yusemi-SWY and T^3 CYR WIZ.
Mega optic ZING!
The first game I ever played with slowdown was Life Force. When I was a kid along with my little brother we used to ace all the Konami ports left and right, we knew nothing about games we just played them (it was more fun back then) and that whole "slowdown" deal we just didn't know what it was, we kept thinking that we were just too damn good dodging all the bullets and the computer or AI couldn't handle our skillz!
We used to call this "El arte del esquive" which means "The art of dodging", it sounded cool and edgy so needless to say we really liked it, we didn't know it was due to the computer getting all cramped up 'cause there was to much moving sprites or something. I like the slowdown it kind of feels dramatic to me, so I do believe it's defenately intentional specially with some of todays shmups. Topday sometimes I think it's a bit overdone though, for some reason it doesn't feel as natural as before.
I wish my little bro could play some cave shooters now days but he's all about GTA and all that current gen mainstream stuff.
BTW, cool thread!