I am not sure if either express your intended meaning, though I agree with Claire that the past simple is much more likely to be correct. We would need to know your context better. You might have meant: "It must have been time consuming."

English has six tenses. Each tense has a PRECISE use. Here are the uses of the two past tenses: Past tense expresses completed action in the past. It doesn't matter when you know it. I saw that movie. I saw that movie three times. I saw it yesterday. I saw it last week. I saw it last year. Past perfect tense is used to describe the EARLIER of TWO past completed actions. By the time he arrived, I had already left. (arrive, leave; leave happened first) By midnight, the carriage had turned back into a pumpkin. (midnight "arrives", turn; turn happened before midnight "arrived")

I would say the second one: I'm sure it was time consuming.