Hello,
Well, for sure it'll take time if that big LOOP (with embedded SELECTs & internal LOOP) is beeing called each time a value is changed 
First of all, all the methods used within the LOOP/ENDLOOP should be moved outside.
Then, try to review your logic in such a way you can work with bufferized internal tables instead of long running SELECTs.
And last but not least, better use ASSIGNING instead of INTO for your LOOP, and forget about the MODIFY statement.
Br,
Manu.