Thunderbird struggles with managing and searching large mail archives in a way that Gmail doesn't. I use Thunderbird for a work email account, and its GUI thread regularly gets blocked while trying to search or open messages.

To be fair, my work email account is an exceptional case - it's got somewhere around ~1 million emails.

Gmail, particularly with the new Gmail interface launched a few months ago, also sucks performance wise, but in different ways from Thunderbird.