I have been struggling with my email shared folders for quite a while now. I always thought the problem is at the server end. After working with my email admin for a few days, we finally found that the problem is related to Thunderbird.
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There is a feature called IDLE command support in Thunderbird. If I have this feature turned on, it’s very slow to access the IMAP shared folders. To turn it off, follow these steps.

A million thanks for your advice. It worked like a dream after a long, tenuous and nightmarish insomnia.