In kmail1 (and I think in previous versions of kmail2) there was a filter field above the folder tree which allowed me to quickly reduce the tree to find a folder. This was either removed or I do not find the checkbox to show it Reproducible: Always
The checkbox can be found in Settings -> Appearance -> Layout, right below the folder list group. The names in the settings might be somewhat different. I guessed the English names from the German names I get here.