What is the best way to approach DI questions? As they tend to be calculation extensive, what is the best strategy to optimise the time?

First of all if you pretty sure that a set is time-consuming and calculation intensive, it is always better not to start with that set. The reason being that if you solve that set 15-18 minutes then it will press the panic button. So look out for other sets which you can solve. Once you are done with all other solvable sets, then you can come back to this set. Now have a look at questions where options are not ver close. In these questions you can use approximations. For the questions with close option choices there is no choice but to calculate.
Thanks.