Instead of defining configuration values, I think it would be better to apply some reasonable values (3 retries, 5 sec. ?) and show an error message when the toolbar doesn't load. I remember that there was an old issue or pull request about this.

My point is that a reasonnable value may not have sens on some situations. In particular, between php5 and php7, the load may be much faster. Or dev on a very slow computer with xdebug and php5, the page may take several seconds to load.

I didn't look into the code in detail but I wanted to note that the behavior of the WDT seems different between the version <=2.7 and 2.8.
With 2.7 the bar may show up correctly but an memory error may be raised on the detailed page. Whereas with 2.8, the bar doesn't load without any php error but a 404.

See #15670 for the prior discussion on this topic. A good solution is mentioned there but nobody implemented yet.