As jrbon mentioned, lots of factors determine this. Taking good medications, excercise and diet as well as avoiding smoking, excessive alcohol can raise the heart function rate.
In my case, I was able to push my heart function rate to close to normal from a heart failure rate, so I know it's possible.
Keep us informed.
It will depend on many factors : Cause of the low performance, age, other diseases...
However, with today´s medicines and devices it is possible to live many years. And if things get really bad there is always the possibility of ventricular assisted devices and transplant.
Jesus