How are you measuring the speed? AIUI, the WD20EARS spins at 5900 RPM.
Can you upload some HD Tune read benchmarks to a file sharing service?
See these threads:
BTW, a drive’s heads fly on an air bearing generated by the spinning platters. If the rotation speed were to vary, then so would the flying height, leading to variations in read/write amplitude. Therefore the heads are aerodynamically tuned to a particular speed, and that speed is tightly controlled. I expect that Thermal Flying Height Control (TFC) could compensate for minor changes, but it wouldn’t make sense to use it for wide speed variations.
That said, I am aware that WDC uses “IntelliPower”. Unfortunately WD’s public information on the technology is no better than marketing-speak:
“For each drive model, WD may use a different, invariable RPM.”