Mask prepend string performance

[Sydnic]

It was md5 hash, sorry for not specifying it. I believe I tried this before for sha512 and it was similar decrease in performance, not entirely sure. Could you help me out by showing me how I could work around that decrease in performance? So i guess i want for example a brute force of all the combinations (of lets say 10 lower case letters) that start with the substring "pass" (e.g. pass?l?l?l?l?l?l as a mask).