Even though everything was below the surface, cutting on something with this much metal in it was still a bit nervewracking.
Default speeds for 1/4 flat, something like 15 plunge and 60 feed, though since I was taking off around .005" I likely could have gone much faster. I switched to some different threaded inserts this time:
I previously used the EZ-Lok brass ones which were technically for hardwoods. They always lifted the MDF a bit, but when I tried this earlier in the week they split the MDF entirely, so I got the correct ones for this application. They’re similar to the ones Winston Moy used in his shapeoko upgrade video but without the flange, so no need to mill out space for the flange to sit lower. Drill with a clutch and a 6mm hex driver got them quickly installed. 14"x14" surface so not quite the maximum for the S3 but close enough for the kind of work I mostly do.
Also I’d been using an older version of CC and finally upgraded it for this. It drilled the pockets full depth in one pass instead of moving around the board in 3 separate passes. Woo! It still drilled the holes in an interesting order and wasted time moving around on the X axis, but it did them all in order on the Y axis so, much better than 3 full passes.