Thanks for the input y'all. I did some further testing and found out something strange. When I change the CPU resources for each app, and it doesn't matter if I increase or decrease the resources and then checking the disk speed again it went up. Also wenn I stress test my website (did that with the siege command line tool) the disk speeds went up. Seems like Contabo does allocate resources when there is stress on the system and decrease resources when it's idling? So I could reach speeds of around 300 MB/s to 1 GB/s on SDA3. Same with SDA2.
I think I'll ignore the numbers for now as I don't feel there are real world problems on the VPS. And I guess if I really need to upscale performance for whatever reason I might switch to Hetzner or something else.