Making Insanes before your first SAsc will slow you down in the long term, skipping them will do it gradually, but the AP increase makes up for it.
Depends on what your current artifacts are; if you currently have 4 Leg artis, then you absolutely want to build that Insane before SAsc, else your chucking 2 Leg
If you have 4 Epics, then just get 1 more and build the Leg and maybe SAsc away
Or just keep on digging while you're in late Phase 4, get 1-2 Insanes while you're digging is 50% sped up
It all comes down to how crushed you are going to feel post SAsc, effectively restarting the game with +25% AP buff, no Adv's & 2 artis; having some Insanes will speed it up a fair bit, and not having them means it's super slow again
If you're okay with super slow, then go for it
But some people just couldn't be bothered actually essentially going back to the start with no significant buffs, and quit the game - in which case you would have been better off continuing in phase 4 until you get to a point where you won't be turned off the game post SAsc