I randomly choosed 4 devices (different boards) to test the flag --send-payload-in-parallel.

All 4 devices are in MTV lab (but I don't know their network condition/routing etc.)
And I run following command on my workstation in TPE.
 $ time cros flash $host xbuddy://remote/R71-11030.0.0/test 
and
 $ time cros flash $host xbuddy://remote/R71-11030.0.0/test --send-payload-in-parallel

==================================================================
host 1
x without
+ with --send-payload-in-parallel
+------------------------------------------------------------+
|  +   +                                                     |
|  +   ++              +                           xx    x xx|
|                                                   |___AM__||
||_____MA_______|                                            |
+------------------------------------------------------------+
    N           Min           Max        Median           Avg        Stddev
x   5     759.68404     788.59561     779.76808     775.46912     12.854345
+   6     606.43274     671.35565     620.91557     624.73901      23.81555
Difference at 95.0% confidence
        -150.73 +/- 26.9988
        -19.4373% +/- 3.48161%
        (Student's t, pooled s = 19.7114)

==================================================================
host2
x without
+ with --send-payload-in-parallel
+------------------------------------------------------------+
|        +                                                   |
|      x +                                                   |
|      xx+                                                   |
|     +xx*                                                   |
|     +*x*+                                                  |
|    ++****x  x               x                              |
|    +******  x  +   x   +    x                              |
| + *+******  x ++  xx   ++x xx   +         +             x +|
|  |_____M____A_________|                                    |
||_______M___A__________|                                    |
+------------------------------------------------------------+
    N           Min           Max        Median           Avg        Stddev
x  39     475.63217     596.94456     486.69279     496.59971     23.862541
+  40     469.13674     602.59352     485.31915     494.06053     26.315035
No difference proven at 95.0% confidence

==================================================================
host3
x without
+ with --send-payload-in-parallel
+------------------------------------------------------------+
|+ ++   +     +      +                              xxxx    x|
|                                                   |_A__|   |
||______MA______|                                            |
+------------------------------------------------------------+
    N           Min           Max        Median           Avg        Stddev
x   6     809.20534     833.29637     815.64993     817.09348     8.5752985
+   6     660.99566     719.19032     682.41796     683.27478     22.025438
Difference at 95.0% confidence
        -133.819 +/- 21.4987
        -16.3774% +/- 2.63112%
        (Student's t, pooled s = 16.7131)

==================================================================
host4
x without
+ with --send-payload-in-parallel
+------------------------------------------------------------+
|   + +                                      x     x         |
|+  + ++     +           +                xx xx    x        x|
|                                         |___M_A_____|      |
||____M_A_______|                                            |
+------------------------------------------------------------+
    N           Min           Max        Median           Avg        Stddev
x   8     960.89797     1040.0684     976.02419     986.21065      25.99646
+   8      778.7987     882.26731     800.51862     809.80691     32.715244
Difference at 95.0% confidence
        -176.404 +/- 31.6896
        -17.887% +/- 3.21327%
        (Student's t, pooled s = 29.5474)

==================================================================
Conclusion: no difference on one device, reduced 2-3 minutes on other 3 devices.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/bisect-kit/+/73e6017f5bdcae864a4a2524a6a60d04f555d415

commit 73e6017f5bdcae864a4a2524a6a60d04f555d415
Author: Kuang-che Wu <kcwu@chromium.org>
Date: Thu Sep 06 15:46:33 2018

bisect-kit: use --send-payload-in-parallel speed up cros flash

By my not scientific experiments on 4 devices, this flag reduced 2-3
minutes for 3 devices, and no difference for 1 device. See detail
numbers in the bug comments.

BUG= chromium:878705 
TEST=switch_cros_prebuilt.py still works

Change-Id: I4c41f9f080e3f7d0dd23aa61f96eb32f1999854c
Reviewed-on: https://chromium-review.googlesource.com/1208126
Commit-Ready: Kuang-che Wu <kcwu@chromium.org>
Tested-by: Kuang-che Wu <kcwu@chromium.org>
Reviewed-by: Chung-yih Wang <cywang@chromium.org>

[modify] https://crrev.com/73e6017f5bdcae864a4a2524a6a60d04f555d415/bisect_kit/cros_util.py