CPU only clocks up to 0.79 GHz?

After a long time, I turned on my laptop again and found that it is very slow and always loads a lot, especially in the browser. It is an I7 4500U and only clocks up to 0.79GhZ and is, according to Task Manager, only 33% busy. CPU-Z says the same and tells me that the processor has only 0.66V.

I'm grateful for any help.

This is normal and can have two reasons

1 the power is not needed and that saves power

2 it's up to the temperatures

long loading times do not necessarily have to be connected with the CPU, as the reason is a slow HDD hard disk