So copying files potentially from a 5400 RPM laptop drive to a faster drive seems limited by the 5400 rpm source drive's speed?
i don't know what are referring to. you tell me you see the results
You have slow 5400rpm internal hard drive. If your laptop is not brand new, there's good chance the files on it are fragmented, which means they will read slower.
There are purpose-build applications to test disk performance.
Observing the Crystal Disk test for both local partitions, it seems that the disks are reporting "acceptable" speed for WD blue-like class of HDs. On the other hand, the external is reporting extremely low speed.
-We can't rule out the fragmentation but testing with both partitions C,D to external would increase confidence that it's not fragmentation.
-The file used for testing should also be 1GB+
-Try different external HDD cable if available
If the problem persists, and given that fact that he's using USB 3, then probably the external isn't working properly and testing it on another machine would be decisive.
Make sure both use NTFS file system format if you're using windows since it usually delivers better speed.