Copying files will quickly speed up to speeds of about 300-500MB/S, but after 5-10 seconds of this fast speed, it will drop to 0-100MB/S for the remainder of the copy. Sometimes even hitting 500KB/S for 30 seconds or so. It will never go up to the high speeds again. Is this normal or is something going on making the speeds drop.
This is normal.
That very fast speed you're seeing at the start is the cache on the drive.
Once that is filled up, then you see normal HDD performance.
Also, it depends a lot on what you are copying.
A lot of tiny files WILL be slower than one large chunk of data.