Why are you refusing to answer the question about how this new column is calculated? Options suggested so far.
1) A view
2) An UPDATE query
3) A computed column...

Are your statistics up to date?

John, Thom, these are not 'calculated fields'. Or perhaps the OP used the wrong term.

Please define the calculation.
How do you get from 'Scenario 1','00 Set',' TC1' to '8:00'?
Does TC1 always correspond with '8:00'? If so, why not just create a lookup table &...

I presume that you are using a table as the source for your data flow?
If so, change this to a query and add whatever filter criteria you need.

This trigger will not handle the case where multiple rows are inserted in a single batch, so you need to perform some re-engineering.
insert table2(id, source)
select i.Id,...