1965 Plymouth Belvedere II

When you say the 1-2 shift is slow do you mean it shifts at a higher speed?
2-3 shift. Just taking off normal, you need to let off the gas to get it to go from 2nd to 3rd. If you don't let off the gas, it will just rev up like it's stuck between gears. 1-2 shift is fine. Even under floored conditions it shifts just fine from 1st to 2nd.