weird. I had the opposite experience. but I'm comparing 25MHz 68k hardware with 40MHz Sparc. I remember it being at least as fast and no spinny pizza disk pointers.
I would be comparing a mono NeXT slab with a SS1 (maybe 1+, not a 2). I do recall having a memory bump on the slab (32Mb?), which probably made a difference. But after 35 years, I may just be remembering through rose coloured glasses and all that. Both were great systems.