Power Query Loading slow after calling a function in custom column

I have created a function to find the working days in Power Query. As soon i call that function in the custom column, the loading of data get extremly slow, like loading 1 row per second (in both data preview and loading). Need suggestions on how to optimize my model. Folloing methods i have tried already,

1) Adding Table.Buffer(dataset) as a step before adding the custom function.

2)  Adding Table.Buffer(dataset) as a step right after the source (with no further applied steps)

Both scenarios gives me an error of A cyclic reference was encountered during evaluation.

Thanks in advance.