Why do many SSDs equipped with M.2 interfaces have quite different speed?

A fast SSD is one of few hardware upgrades that may instantly improve how you experience your PC. This is especially true when switching from a mechanical hard drive, but switching between generations (i.e. from SATA to NVMe/PCIe) may also be noticeable. SSDs are also key components in servers, vastly increasing throughput and efficiency.
Unlike other parts such as a CPU/GPU, a smaller production process as measured in nanometers (nm), is not a strong selling point as it results in lower durability, all other things equal. However, for the average home or office user, durability is rarely an issue, since any SSD will most likely outlive the rest of the computer by a wide margin. Just don’t rely on low-cost drives with cheap NAND in a server or other environment with a high continuous workload
Someone always asks why many SSDs are also equipped with M.2 interfaces, but have quite different read and write speeds. So, what does determine the transmission speed of solid state drive (SSD)?
The transmission speed of solid state state is mainly related to flash memory, master control, interface type, supported channels and protocols.
At present, QLC flash particles are commonly used in consumer SSD in the market. However, even under the condition of using same flash material, its transmission speed still is significantly affected by several other factors. This article will do science popularization from the interface and protocol which are easy to neglect in the process of purchasing for fresh users.

What do M.2 and NVMe mean?
Although many people may be confused with the interface, protocol and channel, they still hurriedly purchase their SSDs. Rich and bitch! Before you make a decision to buy a SSD, you must be sure to understand the basic parameters of SSD. 
M.2, an alternative to mSATA, is a new interface specification introduced by Intel, which is an interface/slot type. Nowadays, the mainstream interface of SSD is SATA interface, M.2 interface and PCI-E interface. There are two types of M.2 interfaces, Socket 2 (B key) and Socket 3 (M key), respectively.
NVMe is a protocol for communication between Host and SSD. In fact, like AHCI, NVMe is a logic device interface standard. Its full name is Non-Volatile Memory Express. NVMe is a specification for SSD using PCI-E channel. SSD supporting NVMe protocol must support PCI-E x 4, but SSD supporting PCI-E x 4 does not necessarily support NVMe protocol.
The master control determines the channel of SSD. Traditional SATA interface adopts SATA channel, whose transmission speed does not exceed 550MB/S. Socket 3 of M.2 interface adopts PCI-EX4 channel with transmission speed over 1500MB/S. If Socket 3 supports NVMe protocol, it can reach 3000MB/S in transmission speed. When adopting SATA channel, the transmission speed of Socket 2 is less than 550MB/S. However, that adopting PCI-EX2 channel can provide a speed of 1000MB/S. The SSD with PCl-E interface reaches 1500MB/S by using PCI-E3.0X4 channel, and 3000MB/S by supporting NVMe protocol.
Many kinds of interfaces and channels mean a lot of mistakes. Therefore, you should be careful when selecting M.2 SSD. Even though it is a SSD with M.2 interface, if it is adopting SATA channel, its transmission speed cannot meet our requirements.
The reason why M.2 interface SSDs are divided into PCI-E SSD and SATA SSD is that they take different channels. M.2 has two interface definitions: Socket 2 and Socket 3. Socket 2 supports SSD with SATA and PCI-EX2 channels, while Socket 3 designed for high performance storage only supports PCI-EX4 channel.
Can a motherboard with M.2 interface use M.2 SSD?
DIY friends may know that solid state drive needs to be connected with the motherboard to perform its due function. Here I want to tell you that even if the motherboard has a slot corresponding to the M2.0 interface, you may not be able to use the SSD with M.2 interface. That is because some motherboard only supports the PCI-E channel, some only support the SATA channel, and some are compatible with both SATA and PCI-E channels.
So before purchasing SSDs of any type, you must first see clearly the interface type supported by the motherboard, otherwise you will probably buy a piece of scrap.
For example, when motherboard interface supports SATA channel, you can return directly the solid state drive with PCI-E interface.
Why do many SSDs equipped with M.2 interfaces have quite different speed?
Here, you can find the answer from the above.
Although it's an M.2 interface SSD, using SATA channel is slower than PCI-E channel. If the motherboard only supports SATA channel, even though the SSD supports PCI-E channel, the SSD will be downward compatible with SATA 3.0. At this time, the speed will not exceed 600MB/S.
Actually, even if both the M.2 interface and the motherboard support PCI-E channel, its speed performance may be inferior to others.
As we all know, PCI-E is also divided into PCI-E1.0, PCI-E2.0 and PCI-E3.0. In theory, the transmission speed of PCI-E3.0 is the fastest. So the following values of PCI-E channel must also be clear.
Additionally, besides the master control, supported protocol and channel, flash memory is also an important factor influencing the performance of SSD. Based on the speed from fast to slow, flash memory particles are divided into four kinds, namely, QLC, SLC, MLC and TLC. Based on speed from fast and slow, flash stacking modes are divided into NAND, 3D NAND and 3D XPoint.
Therefore, when you plan to choose SSD with M.2 interface, you must see which channel, protocol and stacking mode SSD is supporting.
To be blunt, if you have money, you can purchase M.2 interface SSD supporting NVMe protocol and 3D NAND flash. Of course, it would be better if the upstart spend a lot of money on a 3D XPoint stacked SSD. At present, Samsung 970 EVO and Intel 760P series are all wonderful.
But if you are a fresh user, I will advise you to probably choose Intel 660P series. After all, Intel 660P series sold at $59.7 provides M.2 interface supporting NVMe protocol, 3D NAND flash, and a capacity of 512GB.