The best RAM speed for Ryzen 9 5900X is considered to be 3600MHz. Generally, AMD CPUs benefit a lot from faster memory speeds and it is considered good practice to pair a fast RAM kit with a Ryzen CPU. For the Ryzen 9 5900X, a low-latency (CL16 or lower) kit of 3200Mhz or 3600MHz speed memory should be considered as the sweet spot when it comes to RAM. At 3600MHz, the Infinity Fabric frequency is in a 1:1 ratio with the RAM speed which is considered ideal for maximum performance.