newbie question on performance

hi I have zip experience in informix but have a client with a performance problem. I wondered if anyone could decide if the scenario i paint is at all possible with informix online. If the key to a table is a fairly long character string and the first 6-8 characters are usually the same would informix make a mess of hashing the index to the extent that searching the index could end up reading many records rather than a few. My client has run an informix program that is supposed to show the cost of a select type statement and when he uses a wildcard in the key the cost is reputedly 4007 if the select is change to match the first 8 characters of the key then the cost is reputedly 4. Does this mean that the clients app should try not to use wildcards. Thanks in advance for any help. John