Seems like your Lenovo comes with Windows 10/11 "S" Mode which is a very restrictive Windows OS. This OS doesn't allow any 3rd party software except those that comes from Windows Store and only MS Browsers will run on this version.
Now some laptop manufacturer purposely down speed their laptop processor to run at a maximum speed.
But you said that your processor does run normally and then starts to throttle even though, in your opinion, it isn't overheating.
What are the temperatures when it starts to throttle or slow down?
When it throttles check Task Manager under "Performance" to see if your processor is running at 100% load which could be why it is running slow. Also check your Windows drive to see if it is running at 100%.
Also you laptop only has 4 GB of RAM in it. That is barely enough to run Windows which requires a minimum of 2 GBs of RAM for Windows x64 OS.
Check for Malware/Virus infections by running a Anti-Virus scan.
Also open a Lenovo Support and see what they recommend.
You have a very low budget Laptop. I would ask Lenovo if they have put a cap on your processor or not.