Intel has published both data collected from
both users and synthetic benchmarks and the bottom line is that users will experience a real-world performance hit of about between six and eight percent, with systems running 8th-generation processors seeing a smaller impact than those running 7th- or 6th-generation hardware.
Factors can swing the results, for example, 8th-generation systems running SSDs see a smaller impact, older systems with SSDs take a bigger hit, and some workloads, such as complex JavaScript operations, are more badly affected and could see as much as a 10 percent performance hit.
When it comes to synthetic benchmarking, we get a picture that's a little more detailed. Again, the generalization that older systems will experience a bigger hit still holds true overall, but that different workloads can skew the results.