Optimized balloon generation.

Before you call this bloody premature optimization, realize that:
 1. Using number or named values in str.format is the same speed
 2. Using dict() with % is around the same speed
 3. Passing a dictionary with {} syntax to % is twice as faster
 4. Passing a tuple with % is 50 times faster than the first
Therefore, my switch to % is a 50x speed boost, as it avoids a function call overhead.

Signed-off-by: Xiaomao Chen <xiaomao5@live.com>