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[vfxtemka]

Hello?  I have samsung ssd 980 pro and I have aorus x399 xtreme board which is only support pcie gen 3.0.

How ever I can't even get full speed of 980 pro gen4 on my board but, I want full speed of gen 3. I have read that pcie gen 3 x4 max speed is  3.938Gb/s. My SSD connected to my board but speedtest gives me 3Gb/s. If I buy pcie expansion card can it be faster than onboard slot?

Onboard m2 slot or pcie m2 expantion card? which is faster?

[elstaci]

according to Samsung concerning the 980 Pro using PCIe x 3.0:

Compatibility

980 PRO is backward compatible with PCIe^® 3.0. Sequential performances (up to): 3500MB/s for reads, 3450MB/s (1TB) for writes. Random (up to): 690K IOPS (500GB/1TB) for reads, 660K IOPS (1TB) for writes.

It mentions 'Up to" 3.5Gb/s. . If your SSD is working at 3Gb/s it isn't bad at all.

Your are using a ThreadRipper Motherboard correct?

The M.2 Slots closest to the CPU which are controlled by the CPU is the fastest for NVME Type SSDs. The PCIe expansion card probably will not make a difference since it is also PCIe 3.0, In fact it might even be slower than the M.2 slot.

I would open a Samsung Support ticket and asked them if those are normal speeds for your SSD model using PCIe 3.0 M.2 Slots.

[Vynski]

They all advertise higher speeds than you actually get.  If your getting 3Gbs that's pretty darn good.  Check it later on, after you get things installed on it and use it for a while and even those speeds will lower.   Just the nature of any storage device.

[Vynski]

I also have an issue with my XPG GAMIX S70, a Gen 4 M.2 drive.  Mine is limited to PCIe 3 speeds due to the CPU, Ryzen 2700.  Gen 2 Ryzen CPU's are not compatible with Gen 4 M.2 drives and they will default back to Gen3 speeds.

  

[Sam_AMD]

I like the onboard M.2 slot better. Simply put, it does not require any cables (SATA connectors), it's easier and quicker to install, and overall it's faster. (I have not used any SATA, PCIe drives in years).

[wim-bart]

What is better depends on motherboard and NVME drive. But a simple rule for Ryzen is, the closer to the CPU the faster it is. Except for Threadripper with dedicated extra Gen4. x4  PCIe slots always use motherboard slots first. 
Slot 0 is always on a dedicated x4 Gen 4 bus connected to the CPU, less latency. The second one is always connected to the chipset with shared PCIe x4 bus to the CPU, a high performance Gen 4 NVME can influence performance (on short time intervals) on devices connected to the chipset. Other NVME are mostly connected with a mutiplexer or share channels on the chipset.