Once the spring fails there will be alot of chain slack, and shifting will be very problematic. It took me about 5 years until I blew out my first spring in a rear derailleur. After it failed I took it to a bike shop and they told me there was nothing they could do except to  replace the derailleur. 

In the 20 years since the first failure I blew one more spring. Last week I learned there is a quick fix for this problem. I saw a bicycle delivery rider with a string attached to his rear derailleur. See the photo below.

I was confused, so I talked to him about it. He explained that it mostly fixed his shifting issues in spite of a lax spring ( on the rear derailleur ). He mentioned that the shifting was not perfect, but it could never be perfect. He attached his string along a different plane ( as compared to the cassette ). Hence the forces have components that will tug the derailleur into an improper path, negatively affecting the shifting performance.

I must say I am quite impressed with this quick and easy fix. Try it and share with friends.