Mutable vectors are about as fast as it gets (sequential writes to contiguous memory), so within an order of magnitude is still pretty fast. And for the default, catch-all data structure, I'm not convinced that being as fast as possibly possible is ideal. The C++ STL takes this approach, going to great lengths to be as fast as C arrays, but I think it suffers a lot for it in terms of usability.
There is a cultural distinction to be aware of here between Lisp programmers and C/C++ programmers. C/C++ programmers tend to be very "layer oriented." They assume everything will be built on top of built-in constructs like arrays, so they try to make it as fast as possible. But Lisp has historically been more of a "bag of stuff." It offers lists as the default "go-to" data structure, which has a lot of positive qualities. Mutable arrays are a separate data type that you can use if you need the speed. I think Clojure inherits some of the same thinking. Raw JVM arrays are always there if you need them, but the defaults are optimized for things other than raw speed.
There is a cultural distinction to be aware of here between Lisp programmers and C/C++ programmers. C/C++ programmers tend to be very "layer oriented." They assume everything will be built on top of built-in constructs like arrays, so they try to make it as fast as possible. But Lisp has historically been more of a "bag of stuff." It offers lists as the default "go-to" data structure, which has a lot of positive qualities. Mutable arrays are a separate data type that you can use if you need the speed. I think Clojure inherits some of the same thinking. Raw JVM arrays are always there if you need them, but the defaults are optimized for things other than raw speed.