As stated by a couple of others now, small files do seem to transfer a lot slower. the vast majority of my backups, when i first started them, are large files from CD, DVD, and Blu-ray iso rips i've been adding.
Have you tried the drive on a PC for transferring some files and speed tests? Happen to have another cable?
A note about hyper backup. If you do the standard backup, you will need hyper backup to restore files from what i read. I've opted to just do a copy and i have several drives for revisions. It's home use and doesn't change much so it works fine for me. I'm not overly concerned with lots of revisions for backups; more so lose of files. To aid in this, i format the drives connected to windows in NTFS. The synology unit i have reads and writes to NTFS. This way, if i needed to, i can pull up any backed up files directly from the drive on any computer i can connect it to that can read NTFS. The synology unit will not format in NTFS. at least from the web interface. you probably could do it from ssh.
I mention this because some of their other units may be restricted and not even read NTFS (just a guess). I don't know why, but i've seen things mentioned not being available in smaller units.
If you're looking more drives for backup, i like waiting for WD EasyStore 8TB drives to go on sale at BestBuy. you can use them in their usb3 enclosure or shuck them like it do. The EasyStore are BestBuy exclusives so i would avoid buying them from Amazon or eBay. Fair chance someone stripped the drive out and stuck something else in there or is just selling at a mark up. They're currently still on sale for $139.00
https://www.bestbuy.com/site/wd-eas...-3-0-hard-drive-black/5792401.p?skuId=5792401