In a hard drive, over time bits of data become scattered across the entire drive.
Accessing this data and copying it will take time because of the mechanism of an HDD. So yes it is slow.
The construction of an HDD prevents a good access time; it's simply not possible to access random bits of data quickly enough. SSDs on the other hand can do this.
After you finish copying files, if you copy those files again, you will notice a huge jump in speeds because all the data is packed close together and is not fragmented.
Getting a higher rpm HDD won't help.
However, a higher rpm HDD will increase sequential read/write speeds given that the data is not scattered and fragmented.