CPU clock speeds are overrated — this is what actually matters - MakeUseOf

CPUs are mostly advertised by their clock speeds, with GHz being the unit used to evaluate them. While it's important to assess how many cycles per second a processor runs, this metric alone doesn't determine its true capabilities. Other specifications must be considered, as clock speeds mean little without them.

Why GHz became the headline number

The focus on clock speeds began with Intel's Pentium IV processors, which used the NetBurst architecture—a significant upgrade from the Pentium III. These CPUs reached speeds as high as 3.8 GHz, unprecedented at the time. Higher clock speeds became the easiest selling point because buyers could compare a single number rather than numerous specifications.

More than 25 years later, clock speeds haven't advanced much numerically. Modern CPUs like the Ryzen 9 9950X3D run at 4.7GHz, which doesn't seem like a substantial upgrade. The same problem persists today: clock speeds beyond the number feel vague and non-specific for most consumers.

IPC decides what each GHz is worth

Instructions Per Cycle (IPC) shows how much a CPU can do in a single clock cycle. A CPU with higher IPC can handle more instructions per cycle. For example, two CPUs at 4GHz could theoretically handle 8 billion instructions per second, but a CPU with four cycles could handle up to 16 billion instructions per second.

IPC was why Pentium 4 CPUs lost to AMD Athlon 64 chips, as they traded IPC for clock speed. However, IPC isn't a specification CPU makers publish per chip. Because it varies with workload, it's measured using performance counters by dividing instructions retired per cycle. The best way to compare CPUs is through benchmarks at the same clock speed with other specifications locked.

Architecture and cache shape IPC

IPC comes from other CPU specifications and factors. Architecture dictates how the CPU handles instructions, depending on its internal design. Newer architectures can increase core width (how many instructions a core can process), core count (handling more tasks simultaneously), and improve branch prediction.

Cache is another crucial factor. CPUs store frequently used data in three levels: L1 (fastest), L2 (larger and slower), and L3 (largest but still faster than RAM). In gaming, cache improves 1% lows and frame latency. AMD's X3D CPUs outperform Intel in gaming partly due to 3D V-Cache technology that stacks L3 vertically, increasing total cache size.

Cooling and power limits cap real clocks

CPU spec sheets list base and boost clock speeds. Boost clocks are short-term higher speeds when needed, but they raise temperatures significantly, risking thermal throttling when components exceed temperature thresholds and must lower clock speeds and voltage to avoid damage.

This relates to PL1 and PL2 (power limits in watts), where PL2 is Intel's short-term turbo power and PPT is AMD's equivalent. A CPU can only maintain good clock speeds with proper cooling. TDP equals PL1 and is used when buying coolers, but doesn't account for PL2, which should be considered for cooler selection.

Benchmarks beat spec sheets

Clock speeds are unreliable for measuring CPU performance across different generations and brands. To account for all specifications and factors, check benchmarks for the CPU you're considering and compare it to other models in the same price bracket. This gives a better idea of where the processor stands in terms of actual performance.