Colouring 
I knew it, but realised how bad it was this winter when I started working by bigger batches. I have to add here that I do colouring last (after speech bubbles/text).
'Ohhh, great, I have 20 pages ready to colour and that took really less time than expected!'
2 weeks later and 3 pages coloured... 
It's not only that it takes time on itself, it is also that it can become boring so I don't feel like working on it. But effective time spent on it is huge too.
I won't do large batches anymore.