I think that the function (Base Key) greatly reduces performance. Because the search range is too narrow. Now, if it as it to disconnect! We can say that this is a useless function, since one character in 160 changes Sha 256 cardinally.
I have the opinion about this, may be not correct. I guess that the base key function is very important for securiyty reasons. Vanitysearch is a tool designed to generate desired vanity addresses. So these addresses also should be secured - for these purposes random base key is used. Without base key fuction 2 different persons could generate the same addresses if use the same prefix - let's say I generate the address starting with "1Dragon", so another person could generate the same address using vanitysearch without base key.
But anyway, if vanitysearch performance without base key is faster, it will be good be able to disable this function (only for advanced users, as it decreases the security).
PS. Without base key, by default users could use number 1 as teh starting private key. So, all different users will generate the same address if use similar prefix, because the tool increment priv key number by 1 searchig for the address.