It doesn't make sense to call the 102 issue Post-OMD era Brand New Day when it refers to the original arc and that was mostly a tagline than a story title. But that's how it's been grouped.
And Slott wrote most of the issues in the early part of that era.
When you had second series that made sense since as a rule the writers of Spectacular/Web of Spider-Man/Peter Parker Spider-Man/FNSM and so on all worked in the status-quo decided upon by the writer of ASM. So if Bill Mantlo liked Mary Jane and Marv Wolfman broke her up well tough luck for Mantlo.In comparison, even though he still collaborated with Gage & others during Big Time and beyond, he was still the one paving the direction of the story and where it was going to take, and was overall his vision. It's basically like in television, with Slott going from just one of the writers, to the showrunner.
But when all of that got canceled and you had three monthly Spider-Man and later bi-monthly to take the place that distinction doesn't make sense. Slott was one of the showrunners post-BND, and co-author of that.
And since this is about the flagship title and we are comparing the first issues, then Slott's run begins with BND and not with Big Time. I don't see how that's arguable.
As a rule it takes a shorter time to write a comic than to pencil/ink/letter/color it, not including editing. So for a professional comic writer, writing multiple titles and issues, obviously it's hard work to do well, but it's not exceptional. Jack Kirby drawing a year's worth of Captain America comics as backlog before he went to fight World War II...now that's the "hold my beer/that's cute" bar.I do think in terms of both him and Spencer as writers, it is interesting looking at the fairly different ways in how they handle writing. Slott not only seems to love collaboration, but it also seems he works better when he doesn't have too much on his plate, which you can see currently with Whitley, Zub, and Simone being brought on to help with his Iron Man. Meanwhile Spencer is known for reaching Bendis levels of writing scripts super fast, first with how he handled the Cap and Secret Empire books, and now with Spider-Man (April has 4 issues written by him!) There's also the rumour that he had the first 20 issues all written before #1 was released, but I don't think that's ever been confirmed.
Writing ASM twice a month is a little excessive since that's something that's been there since Post-OMD and as a rule it makes stories less economical and encourages more spinning of wheels. Since obviously you have to do events every year, every half-year but the build-up to that is done in more panels and captions and bubbles than before.