It's not a /good/ client, either. It's adequate, but has a long way to go to clean up it's UX and normal reading flow.
That being said, I'm rather happy with K9, and I'd certainly prefer to see energy spent in making it even better than in making a “Mobile Thunderbird” from scratch.
If search works fast on Thunderbird is because it keeps all emails in your local hdd. To be fair you should also set the option account settings > fetching mail > local folder size to 'all' in K9.
Additionally, AquaMail can handle nested folders, which K9 doesn't and never will, according to its developers.