Pet stamina works just the same as player stamina, for purposes of determining base swing speed. Every 30 points of stamina (rounded down), the pet's swing delay will get shorter by 0.25 seconds (making it faster).
The key stamina values for swing speed are: 30, 60, 90, 120, 150, 180, etc... The pet with 135 stamina will attack as fast as one with 120 stamina (or with 125 per your example).
Stayin Alive,
BG