From what I've seen, stack based value types are not necessarily the big performance win they're touted to be. The rule of thumb I've noticed is that, if the struct is much bigger than the size of a pointer, you start seeing a pattern where it's quicker to allocate in the first place but slower to pass around.

I think this is because, on a platform like Java or .NET that uses generational garbage collection, the heap starts to behave like a stack in a lot of ways. Allocations are fast, since you just put objects at the top of the heap. And then, since they're at the top of the heap, they tend to stay in the cache where access is fast, so pointer chasing doesn't end up being such a big deal. On the other hand, if you use a struct, every time you pass or return it you end up creating a shallow copy of the data structure instead just passing a single pointer.

(Disclaimer: preceding comment is very speculative.)