Yeah I can buy a blitzkrieg doing immense damage over mere months, but, as you say, how they're written the Klingon attackers really doesn't square with this. Though when you read the autobiography of Albert Speer it provides fascinating insights into just how internally fractured and generally a complete mess the Nazis were - Hitler encouraged competition among his minions and the "all powerful Nazi war machine" was NOT some well-organised slick machine. Indeed it's rather sobering to think what the Germans might have done if they'd not had such extreme politicking and competition among competing factions.The worst part has nothing to do with the cloak or the time frame. It's that a politically divided foe - basically 24 different houses acting independently - could defeat the Federation. It means that one 24th of the total Klingon fleet is enough to win essentially any engagement with the Federation. It also means the Klingons are not coordinating anything. Not strategy, not tactics, not even logistics. Any admiral worth their salt should be able to use this against the Klingons - to win the war through the Federation's superior coordination.
I still think your main point stands, but I'm just playing devil's advocate here with an example from real world history that turns on its head what a lot of people think they know about how the Nazi war machine worked. It often worked despite a lot of ridiculous inefficiencies, not because it was ultra-efficient and how we tend to think of modern German production processes!