USB 3 Hard Drive not reading/writing at maximum speed

I recently got a MacBook Air and after connecting my 1TB external hard drive (USB 3.0) I noticed that one of my partitions reads and writes slower than the other. BlackMagic reports that my main partition is reading and writing at around 40 MB/s while the other partition has read/write speeds of 80 MB/s. Both are MacOS Extended Journaled. The only difference is their size.

Thanks in advance!