They should do their best for the 1st leg, winning by 2-3 goals difference will be a big advantage for the 2nd leg especially if they can keep cleansheet on the 1st leg. I dont mean to say that Chelsea is not great enough, but I do believe Real Madrid is still a team with higher level than Chelsea. Chelsea might give a big surprise even though the chance is low.
From your point of view, you are right. Chelsea will not play slow football, Real Madrid defenders will have to deal with lots of counterattacks. But I believe nothing is impossible for Real Madrid and Zidane. So Real Madrid's victory with 2-3 goals is difficult but not impossible.