JPG speed /TIF speed

[Sa šo Celarc]

I have original black&white (8 bits per pixel) tif ortophotos  (4500x6000
pixles) with .tfw.
I convert them to jpeg using
gdal_translate  -of JPEG -co "WORLDFILE=ON" -co "PROGRESSIVE=ON"
(and i was happy because the size decreased to 20% of original.

Then I produced two index files using gdaltindex - one for original tifs and
one for jpegs.
Then I produced two .map files - first is using the tif layer with
tiftileindex.shp, the other one is using jpeg layer with jpgtileindex.shp.

Then I compared the speed of response and I realised, that jpeg version is
much much slower then tif version:

This is slow:
http://localhost/cgi-bin/mapserv?map=D:\mymap_jpeg.map&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&layers=ob&width=800&height=800&BBOX=517800,72000,518800,73000

This is fast:
http://localhost/cgi-bin/mapserv?map=D:\mymap_tif.map&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&layers=ob&width=800&height=800&BBOX=517800,72000,518800,73000

Does somebody has an explanation of this? My intuition says that jpeg should
be faster.

Regards,
Saso