I am getting 325MBs write and 400MBs read however my system is slow right now normally they are both at about 500MBs. I do have 512GB SSD so that could be why. I think your speed is not bad considering it's a HDD, my mom has a 2011 MacBook Pro and it's about 50Mbs read and write.
Thats actually a really good score for a HDD. When I ran that test with my WD Blue 750GB HDD, I was getting around 80-90MB/s. The more you fill it up though, the slower it will become.
It will get slower though as you fill it.
Innermost tracks on the spinning disk are about 50-60% of the speed of the outermost tracks. Fill it up to 80% and you will be at 60 MB/s. That has nothing to do with fragmentation just that a HDD starts writing on the long outer tracks and works its way inward as it fills up.