According to the latest unconfirmed Nvidia RTX 4000 specs, the RTX 4090 will supposedly be 60% faster than the RTX 3090, at least in terms of base clock speed.
Hardware specialist Kopite7kimi stated on Twitter that the RTX 4090 could have a base clock speed of 2235 MHz with a possible boost to 2520 MHz. In addition, the insider says that the "actual maximum" speed of the GPU is 2750 MHz, which will likely be reflected when running games.
In comparison, the RTX 3090 base and boost clocks of the RTX 3090 are 1395 MHz and 1695 MHz respectively.