45 MB/s isn't unusual for an HDD, depending on what sort of files are being transferred. External HDDs tend to be slower than internal ones. Don't really see anything that's obviously wrong here. What exactly made you expect 100 MB/s? If that's what the drive is advertised as, those values are always absolute best case scenarios, with real world use typically being a fair bit less (again, depending on what exactly you're transferring).
Well, taking into a account that an external hard drive is a normal hard drive put into an enclosure with a sata to USB 3.0 port adapter, I do worry about such slow speeds. You must be aware of the fact that people are taking hard drives out of their enclosure and putting them into their PS4s and PCs because they are cheaper but perform the same as internal hard drives.
In terms of what made me think I would get 100 MB/s, I always do research before I buy something and I found some indicators that I would get about 100 MB/s.
First, there were about 3 benchmarks that indicated max writing and reading speeds of about 130 Mb/s.
Second, other similar external hard drives in a similar price point, for example WD my passport were tested and got 90 MB/s max read and write speeds.
Third, I timed how long it took for some youtubers to transfer games from internal drive to extended storage in their PS4s and my approximate results were 100 MB/s.
Fourth, digital foundry did some tests with the exact same drive that I have and got faster loading times when compared with the stock internal drive.
Last but not least, common sense, I have had several PCs over the years and 45 MB/s from a hard drive is like the performance we got in 2005.
In conclusion, 45 MB/s from a USB 3.0 External hard drive is completely unacceptable for me in 2019, taking into account that some other people can get better results with the exact same drive I have and also similar ones,