I actually used the Apple Dylan environment. IIRC, it required 48MB RAM -- yes, M -- to launch, which was extremely an extremely large amount at the time -- as in, you probably needed to buy more RAM to run it -- and was itself written in Macintosh Common Lisp instead of self-hosting. UI development? Thing could barely run on a contemporary Mac. Every efficiency claim that was made for it was unproven. Maybe it could have been great, but it certainly needed at least a couple of years more bake time. The NeXT runtime existed and its performance characteristics were understood.
Virtual memory was a huge issue for UI development with a retained-mode system like DPS; thrashing window contents is very not fun if you want a responsive UI. Apple spent years optimizing VM for this purpose after the NeXT purchase.
The interface builder was written in Dylan and running inside the Dylan runtime.
The early use of Dylan (actually its precursor language Ralph) was to develop software on a Mac for an external device, like tablets and handheld computers. The development system was an external mainboard attached to a Mac. Apple released eventually a first device with ARM 610 20Mhz CPU, 640Kb RAM and 4MB ROM as a product. A few of the early internal development environments were written in Lisp on the Mac, using a lot of memory (precious and expensive megabytes!).
> Apple Dylan environment
That was a prototype of the Apple Dylan environment, later released as a technical preview. It was also never a product. Apple at that time often developed software prototypes and the released product was then recoded as a more efficient tool. The technical preview was for the Mac and for develop software for the Mac.
Virtual memory was a huge issue for UI development with a retained-mode system like DPS; thrashing window contents is very not fun if you want a responsive UI. Apple spent years optimizing VM for this purpose after the NeXT purchase.