Hi Gedéon,

This is actually a known problem in Odoo, which I've faced on an evironment before too. The biggest problem is that the time that this takes grows exponentially, meaning that scanning from this view with more than 1.000 lines becomes almost impossible. There are some things that you can do to improve this:

1) You could divide your stock into multiple locations and do inventory adjustments per location - this means you won't get as many lines to scan and that loading times will go down.
2) You can remove the field 'theoretical_qty' from the treeview. Since this is a computed field it will calculate all the lines, which takes up time. Don't forget to let the function also return nothing ({}) in order to have the best performance for this.
You can find the view patch at https://gist.github.com/nim-odoo/137146f2d7a6c3a4b2177558ccaf399b and the patch for the function changes at https://gist.github.com/nim-odoo/31152241a49e8d215ee5e824b47cab5c .
3) Update your Odoo to the latest codebase - since there have been some changes in the calls there is a slight speed improvement seen from this to.
4) Do your inventory adjustments from the "barcode" app (enterprise app). This interface is faster in scanning and handling the lines than the default inventory adjustment views.

I hope this helps you.

Regards,
Yenthe

HOW can this issue still exist a few years later. 

Doing a full inventory is virtually impossible with about 1000lines... 

Spent a weekend counting every thing in the shop to please the accountant, and now its i dare say useless work done. :-|