The OS implements brk under the hood with "swiss cheese virtual memory" so you don't get any real optimizations here. On AMD64 paging is mandatory. It does have a cost but the MMU hardware accelerates this to make it mostly not noticeable.

The brk system call places an unnecessary restriction on how the memory is used that is not imposed by the hardware. Back in the 70s when segmentation was a thing and virtual memory hadn't taken hold this would be different - but virtual memory has won.