Besides std:: cout << is there any other function to send MCU information to the upper computer?

I’m thinking that it’s better to make an information processing system on a computer than to make lvgl. But if data is transmitted through std:: cout, it’s too slow

Good ole printf() still works … but I think you’re worried more about the channel bandwidth than the C++ baggage.
Do you need the data upstream in real time or would post-processing work for your use case?
If the latter, you can store your data into a file on the SD card.
For the former, your best option is to increase the encoding efficiency - instead of sending nicely formatted text strings, treat the data as well encapsulated binary and either send it as such (as long as both sides survive full 8-bit), or preprocess into base64 for fixed, yet reasonable overhead.

How much bandwidth do you actually need?