Very fast for me as well. I just launched it to check, and it took just a second or two to cold launch it.
Takes a few more seconds to refresh and get new messages. I think it's actually faster than checking my email in browser, especially since one of my email addresses is the Yahoo! account I created in the 1990s and use for junk subscriptions.
That new design update definitely made the performance worse. And it looks pretty much the same anyways. Wish I could just revert it to the old one.
What sucks the most is Dark Reader. Previously it worked fine for emails, but now it always takes a second to load. I'll never forgive the thunderbird devs for this "update".