See, dont do similar questions or extremely easy ones. Once you start doing you will be able to distinguish better. Then, sit with a calm mind and in a peaceful exam hall like sitting. dont lie on the bed, use a table and time yourself. When you fix that you want to do 30questions in an hour then only you would get prepared for exams. This activity when you will repeat for a week, it will come in habit and the time you take per question would decrease eventually. Also, dont sit for too long at once. Like in morning, start with boards portion as you would be fresh in the morning and board stuff is a bit more of theoretical than JEE so sit for 2 hours. Do theory and then take a break. Then sit for another 2 hours and do questions. Now once you do two sittings for boards, do one for JEE and practice same chapter from JEE book. Then if time permits, have one more sitting of either depending on need. Take breaks. Bind yourself in a schedule else it will be a problem. Follow it for a week and it will happen automatically then.