.....Going with the assumption that the "meat" of the information is in
the read/write times in the last table.... I ran the diskSpd utility
on several different volumes. C, my SSD drive, M, a Simple (no mirror)
Storage Space volume, D, an ordinary Seagate 2 TB drive, and Y, a
2-way mirror Storage Space volume.

The data:

C:

%-ile | Read (ms) | Write (ms) | Total (ms)
----------------------------------------------
min | 0.054 | 0.098 | 0.054
25th | 0.517 | 0.516 | 0.517
50th | 0.549 | 0.547 | 0.548
75th | 0.583 | 0.575 | 0.580
90th | 0.646 | 0.605 | 0.631
95th | 0.760 | 0.637 | 0.724
99th | 1.863 | 0.812 | 1.575
3-nines | 3.033 | 1.016 | 2.966
4-nines | 3.439 | 1.598 | 3.401
5-nines | 4.952 | 2.656 | 4.839
6-nines | 6.530 | 4.655 | 6.530
7-nines | 6.842 | 4.655 | 6.842
8-nines | 6.842 | 4.655 | 6.842
max | 6.842 | 4.655 | 6.842

D:

%-ile | Read (ms) | Write (ms) | Total (ms)
----------------------------------------------
min | 0.646 | 0.305 | 0.305
25th | 57.019 | 0.999 | 22.134
50th | 117.924 | 4.017 | 86.826
75th | 198.389 | 25.069 | 168.459
90th | 324.357 | 108.540 | 291.209
95th | 447.615 | 143.787 | 414.046
99th | 616.602 | 469.677 | 600.982
3-nines | 759.427 | 747.135 | 751.097
4-nines | 837.450 | 763.727 | 837.450
5-nines | 837.450 | 763.727 | 837.450
6-nines | 837.450 | 763.727 | 837.450
7-nines | 837.450 | 763.727 | 837.450
8-nines | 837.450 | 763.727 | 837.450
max | 837.450 | 763.727 | 837.450

M:

%-ile | Read (ms) | Write (ms) | Total (ms)
----------------------------------------------
min | 0.181 | 0.173 | 0.173
25th | 11.591 | 0.332 | 7.282
50th | 39.781 | 0.724 | 27.151
75th | 232.176 | 85.514 | 179.854
90th | 469.246 | 168.400 | 417.374
95th | 629.072 | 227.541 | 579.232
99th | 1685.823 | 1605.151 | 1663.511
3-nines | 1893.940 | 1866.983 | 1890.843
4-nines | 2107.262 | 1870.728 | 2107.262
5-nines | 2107.262 | 1870.728 | 2107.262
6-nines | 2107.262 | 1870.728 | 2107.262
7-nines | 2107.262 | 1870.728 | 2107.262
8-nines | 2107.262 | 1870.728 | 2107.262
max | 2107.262 | 1870.728 | 2107.262

Y:

%-ile | Read (ms) | Write (ms) | Total (ms)
----------------------------------------------
min | 0.246 | 0.332 | 0.246
25th | 11.666 | 57.782 | 12.136
50th | 24.558 | 378.271 | 36.009
75th | 71.990 | 638.723 | 192.022
90th | 337.932 | 1043.038 | 655.980
95th | 686.719 | 1669.005 | 888.766
99th | 1252.589 | 2724.614 | 2215.368
3-nines | 2544.115 | 2980.434 | 2890.963
4-nines | 2976.618 | 4622.139 | 4622.139
5-nines | 2976.618 | 4622.139 | 4622.139
6-nines | 2976.618 | 4622.139 | 4622.139
7-nines | 2976.618 | 4622.139 | 4622.139
8-nines | 2976.618 | 4622.139 | 4622.139
max | 2976.618 | 4622.139 | 4622.139

This looks pretty bad for Storage Spaces performance... but I need to
run more tests to see if what I'm measuring with 25% writes is really
adversly skewing the results (since I'm primarily interested only in
read speeds).