Some ways you can professionally say time consuming are by using the terms, cumbersome, prolonged, time intensive, lengthy, arduous, laborious or protracted.
When saying time consuming professionally you can use neutral and or descriptive tones which include,
Involved, which is complex and requires effort.
Prolonged which is extended in duration.
Time intensive, which is demanding significant time.
Lengthy which requires a lot of time.
Protracted which is drawn out over a long period of time.
For saying time consuming professionally, using a positive and thorough tone, when it's a good thing.
Resource intensive, which consumes significant resources including time.
Labor intensive, which requires a lot of work and effort.
Meticulous and Exhaustive, implies great attention to detail.
Thorough/Detailed/Comprehensive suggests depth and also care.
For saying time consuming professionally using a negative and challenging tone.
Onerous which is burdensome and difficult.
Tedious which is boring and monotonous.
Laborious/Arduous, which is difficult and requires hard work.