That problem is most often seen on Motorola and Ericsson cell phones. Uniring makes the best it can to convert ringtones to other formats, some of which are VERY limited compared to the Nokring/RTTTL or Nokia formats. One of my articles, ranking the various ringtone formats according to their qualities, should help you understand how they compare.

For instance, recent Motorola cell phones include a melody composer that only features 3 note duration types: quarter notes, half notes and whole notes. These are the slowest note duration types, which means that, if you input a Nokring/RTTTL ringtone with fast notes such as eight or even sixteenth notes, the converter will have no choice but to use the closest note duration. So, when converting a ringtone with eight notes to the Motorola II format, it will have to interpret eight notes as quarter notes (twice as long in duration) and quarter notes as half notes, and so on... That's the best we can do when the target format is so limited and the result is, of course, a ringtone that sounds much slower than the original one.

What can be done about this? Not much. Your best option is to choose another ringtone, one that is already slower, so that the end result does not get changed too much. Some cell phones may allow you to adjust the tempo manually (check your owner's manual for more on this).