The Pentium 3 is what eventually topped out at 1.4 GHz, for the 130nm Tualatin parts introduced in 2001. The Pentium 4 started at 1.4GHz and 1.5GHz with the 180nm Willamette parts introduced in 2000. Those were eventually released with speeds up to 2.0GHz. The 130nm Pentium 4 Northwood reached 3.4GHz in 2004, and the 90nm Pentium 4 Prescott hit 3.8GHz later in 2004.

The Pentium 4 HT 670, released in 2005, came factory-clocked at 3.8 (https://www.techpowerup.com/cpu-specs/pentium-4-ht-670.c20)

Netburst lasted a long time as intel was floundering, before Core Duo was released in 2006.

Intel released a couple of Pentium 4's from different cores topping at 3.8GHz [0].

Tom's Hardware overclocked one of these Northwood Pentium 4's to 5 GHz with liquid nitrogen and a compressor [1].

Those were the days, honestly.

[0]: https://en.wikipedia.org/wiki/Pentium_4

[1]: https://www.youtube.com/watch?v=z0jQZxH7NgM

It hit 3.8 and for a while it surpassed multiple cores' performance because games were built to work on single cores and multithreaded/multicore. It happened with some emulators.

Funny... my pentium 4 ran at 1.6GHz