ASPM is a power management system that turns off the PCIe link to save power. The result is that while you may save power it can actively harm performance.
From Wikipedia Active State Power Management (ASPM)
Currently, two low power modes are specified by the PCI Express 2.0 specification; L0s and L1 mode. L0s concerns setting low power mode for one direction of the serial link only, usually downstream of the PHY controller. L1 shuts off PCI Express link completely, including the reference clock signal, until a dedicated signal (CLKREQ#) is asserted, and results in greater power reductions though with the penalty of greater exit latency.
So when the system thinks the link is even remotely idle it will shut down the data and clock for it.
That shutdown takes time to recover from and to restart the link.
It seems that many implementations in the firmware have this idle detection set too aggressively. As you can see from this related question about power saving modes in Windows it seems that enabling the related setting to turn on ASPM from Windows gives a similar issue that slows down the link and causes odd behaviour that makes the drive seem to be highly active due to far more limited time awake.
There is nothing wrong with your machine, it is simply that the L1 power state is particularly aggressive in putting the link and drive to sleep. Out of preference you should leave the setting on disabled for optimum performance.
If you have the BIOS setting on "Auto", it might be worth checking your power plan settings per that duplicate question to make sure that the link state power management is not set to "maximum". If you set the power management to "moderate" then it should save a little power with far less impact on performance.
To explain the loss in performance it is likely that the speed of the drive is not able to fully saturate the PCIe link on a consistent basis.
A PCIe 3.0 x4 link has a theoretical bandwidth of around 3.5-4GB/s and your drive manages about half that.
The result is that the link is going to be operating in bursts where the drive marshals some data, sends it quickly over the link and then goes idle for a period before sending another burst.
Those bursts may be fast and frequent, but a 50% idle link may well be tricking the firmware into thinking the system is not using it for long periods (long in terms of fast clock speeds) and so in between bursts the drive is sent to sleep. When more data is required the L1 sleep state means that the link takes longer to be brought back up and be usable and that time lost effectively loses you performance.
