Ah, I assume the catch phrase here is "single core performance".
Older CPU designs like the Pentium IV had stellar performance in their primary core,
at the cost of high power usage and heat.
By the time Windows XP and the Pentium IV with hyper-threading (HT) became popular,
games and applications were being optimized for multi-threading and made better use of auxiliary cores.
The semiconductor industry liked this approach, because it more or less ended the MHz/GHz race.
Instead of making the CPUs run stable at high frequencies, they focused on adding more and more cores to a processor's die.
Unfortunately, this doesn't do well with applications that must run in sync with all their sub-routines or which can't be spread accross multiple cores.
Edit: There's something else to consider, maybe.
On a single-core processor, all the caches and the whole pipeline architecture serve a single CPU core.
In a multi-core design, things like cache coherency and parallelism are difficult to handle.
At one point, multiple cores become difficult to maintain and a overhead.
Both for the hardware and the operating system.
"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel
//My video channel//