Some timings (25 October 2019, all on same PC, 4-core 2.5GHz):
All times in seconds.


                                          32-bit   64-bit
USEGDI=1:

  Flat-TZ100-25mm-Flower-916.jpg (6MB):    0.38     0.31
  PanGazer_starter.jpg (11MB version):     0.67     0.55
  Kenilworth-Castle-MFC-2015.jpg (14MB):   0.78     0.60

USEGDI=0, original libjpeg:

  Flat-TZ100-25mm-Flower-916.jpg (6MB):    0.52     0.42
  PanGazer_starter.jpg (11MB version):     0.90     0.74
  Kenilworth-Castle-MFC-2015.jpg (14MB):   1.05     0.82

USEGDI=0, libjpeg-turbo:

  Flat-TZ100-25mm-Flower-916.jpg (6MB):    0.26     0.25
  PanGazer_starter.jpg (11MB version):     0.45     0.43
  Kenilworth-Castle-MFC-2015.jpg (14MB):   0.50     0.48

----

Conclusions:

  GDI is a little faster than standard libjpeg, and has colour
  adjustment (but uses more memory).

  64-bit is similarly a little faster than 32-bit.

  libjpeg-turbo loads JPG files consistently 2x faster than libjpeg