На стекоферфлоу пишут:
The stack is preferred as it is usually in the CPU cache, so operations involving objects stored on it tend to be faster. However the stack is a limited resource, and shouldn't be used for anything large.
Дело в косвенности (indirection), к объектам в куче обычно обращения идут по указателю, в то время как адрес объектов на стеке известен сразу