Cables, adapters, docks, hubs can all affect the speed. Plus other external devices can interfere as can any software issues.
Plus just because an SSD is said to be fast, does not mean it will be that fast. Many SSDs these days can become throttled especially after writing large amounts of data to them. Once an SSD's write cache has been exceeded, you will discover the real performance of an SSD since you are now writing directly to the main SSD NAND storage area instead of a small high speed cache which is used to make the SSD appear fast.
Plus many manufacturers & vendors tend to confuse users with the real speed/performance of their devices. They will advertise what the maximum speed of a particular version of USB is, but that device itself may be incapable of achieving anywhere near those speeds. It is important the user looks for the SSD's true maximum speed separate from the listed USB speed.....many times you will need to look at the product data sheet or at the fine print of the product information. Even knowing the theoretical speed of the SSD itself is not even a guarantee of its real world performance since it is based on utilizing the SSD's write cache for optimal performance.
You cannot even trust any reviews these days. Not because the reviewer did not do a good job, although that is part of the problem, but because the SSD manufacturers may have more than one hardware configuration of their product even though it has an identical part number. Some manufacturers ship SSDs with different internal components....some times at the same time, other times they evolve when another component is cheaper to use. While the SSD manufacturer says they are functionally equivalent & meet the same specs, they don't usually perform the same.
Unfortunately it can be very difficult determining the speed of a device these days. I'm an experienced tech and even I have trouble finding this information these days. Even Crucial no longer provides the actual details in their product data sheets (they used to always provide that information).
