How to decrease capacitance of PIN photodiode
I have used bootstrapping on photodiode amplifiers like this: -



The idea behind it is that any AC the capacitance of the photodiode is largely muted by applying the same voltage at its cathode thus you can use a much smaller feedback capacitor Cf.

The problem comes down to reducing the noise gain of the op-amp when used as TIA (trans impedance amplifier) - because the JFET reduces the capacitance seen at the inverting input, high frequency noise (due to the op-amp) is not amplified as much therefore Cf can be lower and bandwidth can be significantly bigger.


  I am then using an opamp (LM324)


The voltage noise of that op-amp is terrible for applications like this. Choose a device with much better performance and much improved lower bias currents.