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DDR4-4000 CL18 first-word latency is 9.0 ns

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.

First-word latency
9.00 ns
CL18 @ 2000 MHz bus
Row cycle time (tRC)
32.0 ns
tRP + tRAS
Peak bandwidth
32.0 GB/s
Per channel

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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

Peak bandwidth per channel32.0 GB/s

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

DDR4-3200 CL16
CL16 • 3200 MT/s
View details ➜
DDR4-3600 CL16
CL16 • 3600 MT/s
View details ➜

Frequently asked questions

What's the true latency of DDR5-6000 CL30?
True latency = (CL × 2000) ÷ speed = (30 × 2000) ÷ 6000 = 10 ns. That's one of the fastest mainstream kits currently shipping; most DDR5-6000 kits at stock JEDEC timings sit closer to 13-14 ns.
How do CL and MHz affect RAM speed?
Data rate (MHz) determines how much bandwidth you get; CL determines how long you wait for the first bit. Higher MHz + lower CL = more throughput plus faster access. A kit with twice the MHz and equal CL halves the true latency in nanoseconds.
Is DDR5-6000 CL30 better than DDR4-3600 CL16?
DDR5-6000 CL30 is 10 ns true latency; DDR4-3600 CL16 is 8.9 ns — DDR4 is actually slightly lower latency on that comparison. But DDR5-6000 has nearly 67% more bandwidth, which matters far more for modern CPUs, especially AMD Ryzen 7000/9000 and Intel 12th-gen+.
What are tRCD, tRP, and tRAS?
Secondary timings. tRCD is the delay to activate a row before accessing a column; tRP is the time to close a row before opening another; tRAS is the minimum time a row must stay open. All three are measured in cycles; converting to ns uses the same formula as CL.
Does faster RAM help gaming?
On AMD Ryzen and modern Intel platforms, yes — especially in CPU-bound 1080p titles where the gap between DDR5-5200 CL40 and DDR5-6400 CL32 can be 10-20 fps. At higher resolutions the GPU becomes the bottleneck and RAM speed matters less.
What is JEDEC vs XMP/EXPO?
JEDEC is the default SPD profile every DIMM boots with — conservative timings guaranteed to work anywhere. XMP (Intel) and EXPO (AMD) are overclocking profiles stored on the DIMM that you enable in BIOS to unlock the speeds printed on the heatspreader.

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