Sometimes the difference is more than an order of magnitude
Intel Xeon processors of the Emerald Rapids generation appear to provide huge performance gains when activating AVX-512 instructions, and the gains are almost free in terms of power consumption.
The authors of the Phoronix resource tested the flagship 64-core CPU Xeon Platinum 8592+, comparing the performance with and without AVX-512.
Of course, the results are highly dependent on the application. In some places the increase was quite insignificant, and in others it was more than 10-fold!
On average, AVX-512 gives an increase of approximately 100%, which is incredibly high. Moreover, the difference in power consumption is minimal: 583 W without AVX-512 and 590 W with AVX-512. These are average values, and sometimes the peaks differ more, but still, AVX-512 activation used to be much more expensive in this sense.