Lenovo ThinkPad T16 Laptop being very slow when Fan kicks on.

Your laptop configuration is decent. It should not be as slow as you mention. You mentioned about opening a lot of browser tabs. I guess that is the problem. Browsers consume RAM a lot.

Close the browser and then check for the system speed. Also make sure the OS is updated.

If it's an Intel processor, then it will slow down if there is excessive processor heating.

A better guess though is that what is going on is that a power management setting is throttling the system.

First inspect the power setting section in the BIOS then also inspect the power management settings in the OS and adjust settings to suitability.


Added Note:
Oh ya, often (particularly with vendors like Lenovo), they will have included their own power management utility with the OEM installation. Therefore, check to determine if such utility is also operating in force. (Personally, I dont like 3rd party utilities like that as they can cause conflicts; ie, too many cooks in the power management kitchen; see it frequently happens with sound/audio.)

Lenovo is an overrated brand that is basically Acer level but with no stigma. Thinkpads are riding on name more than substance these days.

But does it happen with just one tab of FB messenger or a bunch of them, and which browser? Is tab sleeping on?