Result is instant because you know the result, this is called a verification. Otherwise on average you need precision to represent 2**n values for each range. In reality the precision needs to gradually increase according to log-scale rules. In even more reality floating points can't hold any rational numbers, just close-enough values (think about 1/3 which is impossible to represent as a non-fraction by a binary-based computer), so most likely you'll skip the value you're looking for unless you know its floating point formula (based on mantissa and exponent) can represent it exactly. High-level decimal libraries expansions can't save you from these problems.