A 176 Uncertain

The rule as written does not seem to anticipate that possibility -- it is unclear on how multiples of 5 feet of speed (but not multiples of 10) should be handled. While there are several different reasonable interpretations or ways to adjudicate this rule, a hard-and-fast RAW answer may not be possible.

The most RAW-like answer I can give is to go back to the default rounding rules, which would say that since 25' is not a full 10' slower than 30', it rounds down to zero 10' increments less than 30', and therefore no penalty. (Likewise, a 15' speed would lead to only a -6 penalty, not -12 under that interpretation.) Without a clause to the effect of "or any portion thereof", I believe the rounding rule takes precedence from a RAW standpoint -- it clearly fits the intent for speeds greater than 30' (e.g., you wouldn't get a +4 bonus at 35', because you're not a full 10' faster than 30'.