When I’ve tried 1" slabs under speakers it made the sound brighter and harsher, which I didn’t like (yes, this was a lazy ham-fisted attempt with just hockey puck footers). Didn’t find it worthwhile under components either, and they’re a PITA to handle at that size / weight.
However HRS has used thin slices of granite in combination with dissimilar materials (constrained layer damping) to apparent success - but that’s quite different to a singular slab of granite.
You should consider using it in conjunction with something like sheets of Herbie’s grunge buster or generic sorbothane to control the ringing. Full coverage (thin sheet) is much more effective than just footers.
I try to avoid speculative theory, but I believe the primary resonance frequency of the support is more important than the speed of propagation in the material. And this particularly true in turntables where the suspensions are tuned to different frequencies.
This gels with my experience - the interaction between support structure (rack) and turntable suspension / plinth is going to matter a lot more than anything else, for some systems. Those on concrete slab can get away with much more here. I’ve had turntable & rack combos resonate in the audible bass range (really bad), in the subsonic "woofer flapping" range (REALLY bad - most rumble filters don’t attenuate this enough), and in the relatively safe sub-2Hz range (SOTA).