Therefore, for multi-threaded tasks it will be much slower than Zen 3.
However, it is expected to have an IPC comparable with Ice Lake and Tiger Lake, so if it would reach 5.4 GHz, it might be a little faster than Zen 3 for single-threaded tasks and gaming.
Looking historically, the 6700k reached 4.2GHz, the 7700k 4.5GHz, the 8700k 4.7GHz, the 9900k 5.0GHz, and the 10900k 5.3GHz - which would imply 5.5 or 5.6GHz for what will (presumably) be the 11900k.
It's not like Intel can make 5.4 GHz chips for days on. It doesn't work that way. Ofcourse they can manufacture processors capable of some ridiculous 6.0 GHz (not accounting for power consumption)... but that would be 0.1% yield. You cannot run a factory like this.