You know I was wondering, when I try to search for 7 char prefix by entering only 7 characters, and by searching for the full address I get the same speed and performance.

Unless programs such as vanity and bitcrack can magically turn 7 characters into a complete rmd160 hash and just search for the hash instead of address, then I can't see a reason as to why searching for full address and 7 char prefix would have the same speed.

According to you those tools decode the address we give them and then only search for the rmd160, decoding the prefix of an address gives no clue about the actual rmd160 hash of said address, therefore they all are hashing the rmd160 twice with sha256 algo in order to find the correct checksum, hence slowing down the process.