I have installed 11.0.1 Big Sur on two different computers (fresh install), and I noticed that for both of them, the "Speak Selection" functionality works only with a significant lag. I have tried both English and Chinese, and it only happens in Chinese for the "Siri" voices - e.g. Siri Female, Mandarin; Siri Male, Mandarin. It doesn't work for the "non-Siri" Chinese voices, such as Ting-Ting, nor does it appear for the English versions of the Siri voices. It didn't happen previously with Catalina.
But in cases where it does occur, the lag can be 5-10s before the voice would after after I highlight the text and press option-esc
But in cases where it does occur, the lag can be 5-10s before the voice would after after I highlight the text and press option-esc