Is DDR4-4000 CL18 worth it? Latency, Infinity Fabric, and setup guide
DDR4-4000 CL18 is where DDR4 taps out for most mainstream CPUs. The 9.00 ns first-word latency is only marginally better than DDR4-3600 CL16, and on Ryzen the 1:1 Infinity Fabric ratio breaks, meaning this speed usually underperforms DDR4-3600 CL16 on AM4 despite the higher data rate. On Intel 10th-gen+, it's a legitimate speed grade.
Calculator
RAM Latency Calculator
Timings are in memory clock cycles. True latency in nanoseconds is derived from the data rate.
True Latency
First-word latency (CL)
9.00 ns
Time from a read command to the first bit of data arriving.
I/O bus clock
2000 MHz
Cycle time
0.500 ns
tRCD delay
11.00 ns
tRP delay
11.00 ns
tRAS
21.00 ns
tRC (tRP + tRAS)
32.00 ns
Assumes one 64-bit channel. Dual channel doubles this, quad channel quadruples it.
How this is calculated
2000 MHz bus clock, 0.5 ns per cycle, 18 cycles of CL = 9.00 ns. Per-channel bandwidth is 32 GB/s, 11% over DDR4-3600. The CL18 loosening almost completely cancels the data-rate gain, leaving a tiny 1% latency advantage over DDR4-3600 CL16, and only if the memory controller can run the higher speed stable, which many AM4 chips cannot.
Verdict
DDR4-4000 CL18 is an Intel-only speed grade in practice. On AM4 it's slower than DDR4-3600 CL16 because of the Infinity Fabric break; on Intel 10th-gen+ it's a valid small step up, but rarely worth the kit price premium.
More DDR4 scenarios
Frequently asked questions
Why is DDR4-4000 CL18 bad for Ryzen CPUs?
What is the first-word latency of DDR4-4000 CL18?
Related tools
Data Read Visualizer
Visualize the massive speed difference between CPU cache, RAM, and storage.
Use tool ➜Data Transfer Calculator
Estimate transfer times for files over USB, WiFi, Ethernet, and more.
Use tool ➜RAID Calculator
Calculate usable capacity and fault tolerance for RAID 0, 1, 5, 6, and 10.
Use tool ➜Power Cost Estimator
Estimate annual electricity costs for your PC, Server, or TV.
Use tool ➜