Formatting the HDD does not decrease the performance lol.
Unless if you are nuts and use GUTMAN method to erase the HDD lol.But I am pretty sure you don't do that
Performance depends on a lot of things,not just on formatting.For example:
-if your HDD barely works and you have many bad sectors on the HDD,that will decrease the read/write speed and cause performance decrease...
-if your RAM amount is very small and you use a lot of stuff,(especially in the same time),PF usage is increasing ,(unless you have disabled it completely), and causes the HDD to do a lot of work and that causes a peformance decrease...
-if your processor is weak and you are doing a lot of things that needs strong CPU such as long video encodings,that will cause the CPU usage to jump very high if not to even 100%,(depending on the processor strength),and will cause a big performance decrease...
-if OS's registry is full of invalid entries or it is really messed up,it will cause performance decrease...
(Same applies for corrupted OS data (files))...
-Sometimes overclocking can cause performance decrease...
-if CPU temperatures are very high,it will cause performance decrease...
-and so on...
If you have a bad performance,the reasons can be MANY.Instead of formatting HDD and installing Windows operating system very often,rather try to find out what causes performance decrease.
Cheers!