@OP, the problem with your speed is that you're not taking advantage of your M.2 ports with a SATA3 SSD (6gb/s). You really need a PCIe x4 SSD if you want better boot speeds (32gb/s).
@OP and @dmdilks, it doesn't matter one bit whether you use M2H_32G or M2D_32G slot. In this case I recommend using M2D_32G because as a SATA3 SSD you will only be losing one SATA3 port, SATA3_3, and its corresponding SATAe is disabled. If you stick it into M2H_32G you will lose an entire PCIe x16 slot, PCIEX16_3, but its the slow one with only x4 lanes of PCIe. Either way will work however and will not impact system performance in anyway shape or form though since you're not using either of those things at this point. There are more than enough PCIe lanes to cover your video card, PCIEX16_1 and your other SATA devices, SATA3_0 through SATA3_7, minus that one SATA3_3 if you use M2D_32G.
@dmdilks, there are zero compatibility issues with a SATA3 SSD in an M.2 slot with Windows 7 64 bit. You only get into compatibility problems with Win7 if you are trying to setup an NVMe SSD.
@OP, speaking of different types of PCIe SSD's, if you do plan on upgrading I suggest getting an NVMe instead of an AHCI SSD. They will actually take full advantage of the 32g/s speeds of the M.2 PCIe Gen3 ports on this motherboard thus providing you with the absolute best possible boot speeds. Also if you do go this route, at that point I recommend using M2H_32G over M2D_32G because screw losing four SATA3 ports over a useless PCIe x16 slot that runs in x4 mode.
