Time it takes to open a session file in nogui mode:

                       3xf5     1tr4    5ijn     3j3q
number of atoms        3687    68540  594304  2440800
file size (MB)          0.7      6.6      80      254

compressed pickle     0.041     0.25    1.67     8.24
    \ file size        144K     1.6M     14M      54M
    \ save time         .12      1.3     9.5     30.5
uncompressed pickle   0.039     0.21    1.30     6.85
    \ file size        652K      11M     96M     379M
    \ save time        0.04      .18     1.4     11.4
for compressed files:
  deserialize data    12.2%    33.4%   56.0%    52.1%
  resolve reference   15.1%    17.6%    1.6%     0.1%
  restore data        13.2%    34.1%   40.4%    47.4%

compressed msgpack v2 0.040     0.22    1.49     7.17
     \ file size       143K     1.6M     14M      54M
     \ save time       0.13      1.4     9.7     31.8
uncompressed msgpack  0.038     0.18    1.14     5.82
     \ file size       628K      10M     93M     366M
     \ save time       0.06      0.2     1.7     12.1
for compressed files:
  deserialize data     6.2%    28.3%   48.1%    42.7%
  resolve references  14.8%    17.2%    1.7%     0.1%
  restore data        13.8%    33.3%   47.4%    56.5%

msgpack v2 % faster than pickle
                       2.5%      14%     12%      15%

msgpack v3 with Python decoding of extension types:

compressed msgpack    0.144     1.88    16.8     65.8
     \ file size       143K     1.6M     14M      54M
uncompressed msgpack  0.144     1.84    16.4     65.6
     \ file size       627K      10M     93M     366M

msgpack v3 slower than v2
                      3.60X    8.55X   11.3X    9.18X