We have a friend here who does comission work. He works like an assembly line, and has very little issue meeting his deadlines. Then again every now and then someone pays him to 'go nuts'. Make a galaxy pattern, try wet blending with the new colorshift paints, your call, I dont care. So he takes as long as he needs to because thats what the customer wants: a unique, original result thats closer to art than car paint. But for 'skaven army Nš 259', its all airbrush and mechanically going through the paces.

Maybe its a culture thing, but deadlines and creativity dont need to be at odds with each other. You say there are 4 weeks and 3 people for a book, right?. You already have data and work from the indexes. You get 3 days to come up with random crap. Creativity is not spontaneous, its far easier to be creative within a set framework than to 'come up with something' starting from a blank. You've got years of design work to lift from, between ability names, past iterations, past interactions, tons of result that avid fans have already sifted and streamlined for you for free. So after this 'run wild' period you begin to fill pages. Grab your target pagecount number, slash the portions reserved for artwork (like on ANY publishing work ever, this is not some secret science, even college journals work this way), stablish word count per section and set to filling them up. Army rules go here, subfaction count is 6, so 2 for each people, bring them tomorrow we see if any steps on any other's design space. Again, plenty of material to work with, from fiction to videogames to past editions to RPG books. Too much? jump over to Lexicanum / Warhammer wiki were generous nerds already do your 'hard' work of data collation for you for free.

At the end of the week, all non-datasheet work has no excuse to be over, and thats being extremely generous with time. Datasheets already also have a lot thats done for them.

Take Eldar:
- Infantry is T3 S3 W1 Mv.7 WS3 BS3.
Thats your base statline, tweak it as fits battlefield roles (speedier banshees, tougher wraith constructs, etc). Thats less than a morning for what? 12 statblocks? across 4 people? hell make it a half day per battlefield role, thats 3 days, another day to go over each with special rules (which will also lead to copy paste; all bikes get 'always advances 6', all shuriken get 'high AP on 6 to wound' etc. then another for a final review. Thats 2 weeks. Another week to tweak point costs and another to playtest. Where is the pressure?