Today, there are three main types of external drive interface. They are:
- USB3.0 - the speed limit of USB 3.0 is 625MB/s. USB3.1 also goes up to 1.2GB/s. You might also have an older USB2.0 external but that’s unlikely, but if you do that’s limited to 60MB/s
- eSATA has a bandwidth limit of 600 MB/s
- If you are using a Thunderbolt 3 external drive (very rare and not many PCs have a Thunderbolt port — this is an Apple thing) it has a maximum of 5GB/s speed
Internal drives connect nowadays either through SATA or NVMe.
- Internal SATA SSD has same speed as eSATA and tops out at 600Mb/s
- PCIe NVMe, which has a maximum data rate of 4GB/s
If you are using USB as your interface for your external drive, it’ll generally be slower because the USB bus uses some CPU resources and this can bottleneck compared to SATA and especially PCIe. You generally want to avoid using USB for storing things that need to run quickly while your system is underload. SATA is similar but not as bad. PCIe uses less resources and doesn’t mess with your CPU so it is a lot better.
If you are using Thunderbolt or USB3.1 you are capable of reaching faster speeds than an internal SATA drive, as long as the SSD itself has the speed to match the USB3 or Thunderbolt 3 specification.
In general SSD drives are not fast enough to max out the Thunderbolt or USB3.1 bandwidth; SSD drives currently top out at 3.5GB/s (most drives are not even this fast).
So in short, if you are on Thunderbolt 3 or USB3.1, the external SSD can exceed the speed of an internal SSD connected through SATA, and will more or less match the speed of an internal SSD using NVMe.
However, the external SSD will still be using the USB or Thunderbolt bus, which uses up a little extra overhead, compared to the internal SSD on the PCIe bus, so the internal drive will still win out in performance in general.