I am not sure I fully understood. I have to add spaceless tag in the attributes block? or around every call of this block?

@snoob no. {% spaceless %} is about removing all spaces between HTML tag in the output, which is not what we want (and has a performance impact, which is why we stopped using it). I asked about using Twig whitespace control to ensure that the spaces added for readability of the template don't end up in the output: http://twig.sensiolabs.org/doc/templates.html#whitespace-control

btw, this should not be done in the master branch (the master branch is the 3.0 branch, and the only work we do in it directly are the removal of BC layers which are breaking BC for 3.0). It should go in the 2.8 branch (which is the default branch of the repo btw)

I already proposed these changes but it was declined for performance reasons: #12148 (comment)

@stof : Ok thank you
@Tobion : I was wondering about performance impact also. I tried to search if someone already proposed these changes but i didn't find it. May i close this PR so?