We show that the inscription velocity of fs-laser written structures in YAG crystals can be significantly improved by the use of MHz repetition rates for the writing process. Using a 10 MHz inscription laser, record high writing velocities up to 100 mm/s are achieved. Also, the selective etching process is accelerated using a diluted mixture of 22% H₃PO₄ and 24% H₂SO₄. The diluted mixture enables selective etching of up to 9.6 mm long, 1 µm wide and 18 µm high microchannels in 23 days. The etching parameter D of 11.2 µm²/s is a factor of 3 higher than previously reported and the selectivity is even increased by an order of magnitude.