Hello Srinathy,
The most common reasons are:
1) Huge volume of data
2) Aggregation (Sum/Count/etc - basically everything which requires Group BY)
3) Complex filtration ( using multiple If / Not / AND/ OR conditions)
4) Using If() in a field usually slows it down but not tremendously
5) VLOOKUP is extremely slow function.
Try to put exit scripts in between steps and see which one runs the slowest. You can identify the problem based on what step takes most time.
I hope that helps!
Kind regards,
S.T.
