Devel issue - "Rebuild the theme registry on every page load" won't work
Not a straight solution and not really the best one but it seems that the co-problem; clearing the cache after every css change, was mainly server-sided, rather than Drupal sided --- I store at siteground.com and it was a Cpanel tool named "Supercacher" that caused all this mess and apparent lifetime wasting (and yet it raises the chance for good SEO when you site is finished).

Anyway, When you develop, better turn it off in Cpanel. 

Btw, this was enough by it's own for me... Neither Devel nor any other external module is needed for this solution to work... Just disable the tool at Cpanel.