If you really want to optimize it, start with the lowest allocation size, run a test without compression and time it. Do the same with the next allocation unit size up. You’ll likely see a trend where the smallest ones are slower, with bigger ones netting more speed - up to a point. Then it will stay about the same. That point is what you want. Larger allocation units waste space. But the speed boost in the sweet spot is worth it.
Restic may not be fast enough to saturate the speed - if so, use something like CrystalDiskMark instead.