What I noticed today is:
If the RPM is more than 4K. and you press clutch before applying brakes then the RPM comes to its normal position slowly and does not drops. But if you apply brakes without depressing clutch and when the car is slow enough and you need to shift to lower gear, you press clutch and RPM drops below normal position (in this case when you press clutch, RPM is usually below 2K).
During day time driving it is negligible but during night time driving when headlights, fog lights, defogger etc are switched ON then this problem is more evident and RPM stays below normal position longer. Today I tested this during day time with headlights ON, combination of headlights + fog lights, headlights only on high beam, fog lights only, all lights + AC ON etc. and the drop issue was there when any one of the lights was on.
So I think the problem is due to smaller battery. I will contact Toyota Dealership for this and let's see what they say.