We often see people comparing ssd based only on their max advertised
speeds like 2000, 3000, 7000MBps etc without realizing that in most
scenarios this is practically meaningless for a typical user. To understand these speeds one first need to understand basic classification of speeds of ssd/hdd which is as below:
1. Sequential speed: This is the speed which you get when copying a large file(say 1gb video file).
2. Random speed: This is the speed which your operating system(windows/linux/android) uses(aka boot, restart, software open etc) & is also used when transferring large number of small size files(say a folder with hundreds of photos each with size around few hundred kb or mb).
Rule of thumb is, sequential speeds are always much more than random speeds.
Think of it as a Ferrari with 0 to 100kmph in 5 seconds given a 1 km road in case of sequential speed & a 20m stretch in case of random speed & you can imagine the avg speed achieved by Ferrari in both cases. For Hindi users it can be more humorously explained by below meme.
Basically, a device can never utilize its full speeds for random speed because by the time the device would have achieved its max speeds the task is already finished so again start from zero.
Now the second most important thing to consider is, Bottleneck.
Imagine you are waiting at a traffic signal for green light & in front of you is a manual non-motor rickshaw with no space to overtake it from side. Once the light turn green the rickshaw start moving at 10kmph & your vehicle will also need to move at same speed to avoid collision until that rickshaw move out of your front or you get some side space to overtake it. In this case rickshaw is your vehicle’s bottleneck. Similarly in case of any device speed achieved in a system/process one need to take a look at what is the bottleneck because no device speed can be more than the bottleneck in the system/process.
Most people have laptop/desktop with hdd or sata ssd so using them as source/destination with a 5000MB/s NVMe SSD will result in them being the bottleneck & one will never see more than 150MB/s(typical HDD max sequential speed) or 500MB/s (typical SATA SSD max sequential speed) during transfer. To see the 5000/7000MB/s speed during transfer one need two of such NVMe ssd installed within the same pc(laptop/desktop) with one acting as source & other as destination to remove the bottleneck.
So to conclude, don’t buy a SSD just based on its max advertised speed which is always its max sequential speed. Decide based on your usage(more sequential data transfer usage or more random data transfer usage) & the bottleneck in your system(system already having HDD/SATA SSD or only 1 NVMe slot).