Recently I bought an SSD drive (Samsung Evo 860 250GB) and installed it in place of the DVD drive (SATA II 3.0Gb/s). I removed the DVD drive. Then I used Samsung Magician and copied the HDD drive to the SSD drive. Everything works perfectly. But now I would like to swap the HDD with SSD because currently my SSD can reach a maximum of 250 MB/s where according to the specification this SSD can reach 550 MB/s. Can I swap the hard drives and keep Windows running normally? (I have windows 10 installed). Or do I have to format the drives and install the OS again?
You shouldn't use the DVD connection for your main disk. You would get much better performance if you exchange the placements (and connectors) of the two drives.
If the SSD is a good bootable copy of the HDD, then you wouldn't (hopefully) even need to change anything in the BIOS or to re-install the operating system.