Is force due to electric field instantaneous? If not then why?

No two things in the universe happen "instantaneously", unless they are at exactly the same location, because "instantaneously" would have different meanings for observers moving at different velocities. Maxwell's equations, which describe electromagnetic interactions perfectly for most practical purposes, contain time-dependent terms that describe the propagation of changes in an electromagnetic field. If your Q is moved at all, whether fast or slow, the resulting change in its field at a distance D does not occur until a time t = D/c. That is, the change propagates out from Q at the speed of light. This is an observable fact that, per special relativity, is the same for all observers.

If you accelerate Q, then you will create a changing electric field, which creates a changing magnetic field, which create a changing electric field, etc., and the result will be EM waves emanating from Q, and they will travel at c to q, changing the force on q when they arrive.

But if there's no acceleration, there's nothing to react to. SR says that there is no such thing as an object moving in an objective sense; there is some frame of reference in which Q is stationary and the electric field is fixed. q will experience a changing electric force as it moves through different parts of that field, but the field itself remain constant, and there is nothing to propagate.

It is not instantaneous.According to Lienard Wiechert potential, where one can see that the effect of the charge does not travel faster than light.So force doesn't acts instantly.