[HO455]

The stock turn signal system's flash rate is determined by the resistance of the system. The flasher has a bimetallic strip inside that the current to the bulbs flows through. When energized, the strip heats up until it moves and breaks the circuit. Once the circuit is broken the strip cools moving to the original position where it reconnects the circuit making the bulb come on until the strip reheats then the bulb goes off again. The greater the current draw the faster the strip heats up and breaks the circuit. Which is why when trailer lights are added to a stock truck the signals go into hyper flash. More load equals more current equals more flashing.
Your new bulbs have greater current draw than your old ones did and the flasher is operating at a faster rate due to the increased current. The new bulbs are probably brighter than the old ones. Going to an electronic flasher should solve your problem. They are common, inexpensive, and the fix for trucks with trailers.