Download speed problem in macbook air M2

It depends on the health of the Internet between your modem, your ISP and the server from which you were testing your bandwidth. That health is a very dynamic thing and if one of the edge routers at one of those Internet hops is configured wrong or failing, that might have a significant bearing on your measured bandwidth. If your Internet service is on a cable provider, too many people performing large bandwidth activity on your cable segment will slow it down considerably. And if you exceeded your ISP's usage capacity, they may have throttled your Internet bandwidth until the next billing cycle.

Talk to your ISP and see if the agree that the health of your network connection from them to you is OK.

Restart your modem and then your router and retest.