Acer laptop 0.39 GHz, why (2.3 GHz base)?

I have an Acer Aspire 3 laptop that is fairly new, it cost 800 euro and is actually always very fast, never a problem with it. But when I turn on the laptop sometimes it is very slow. I open the task manager and the performance is 0.39GHz, today it was like that all day, I also tried to restart the laptop, it didn't work. Why is that so?

Look whether a throttling is set in the BIOS / UEFI.

I have no idea what that means, will you explain it please?

Then, unfortunately, it becomes difficult.

When starting the computer you have to press either backspace, F1, F2 or F10 within the first few seconds (depends on the manufacturer of the laptop).

Then you come to the BIOS / UEFI menu.

Depending on the BIOS / UEFI, you can check the CPU settings there to see whether there's a limit to the clock rate of the CPU.

If you are not familiar with it, you better ask a friend who is familiar with PC's.

My laptop is on my desk, there's a heater behind it, but it is not used. My laptop is rarely taken away from the table. Always stands in the same place

If there's a heater behind the laptop, I assume there's a window there too. Depending on its orientation (cardinal direction), the laptop may get direct sunlight, which can also cause a thermal problem.

What can I do to make it all right again?

If my assumption about the sun exposure is correct, you could simply move the laptop so that it is only operated in the shade.

If necessary, you can put something between the window and the laptop as a sunscreen, but this may interfere with the view outside and ensure that a little less light enters the room. Wouldn't be my first choice.