IR led extremely slow to turn completely off?
Your problem is for your Opamp.
As I see, your RC value is 100nF and 1MOhms is 0.1 seconds, and your plot is a nice 1/0.1s+1 plot.
So everything is OK based on your circuit. if you want to increase speed, lower C or R to get a better speed.
Your 500 ms delay is  a 5 time of time constant of this circuit, which is also shows me that it's a first order transfer function. your exact equation in time based system is 1 - e^(-t/RC) where R and C is your capacitor and resistor in your Opamp.