Getting "Installing..." for too long, during any app installation attempt
Best attempts to make your phone fast and long lasting is using some powerful apps which work only of your phone is rooted. Root your phone and download apps like greenify (for battery purpose), roehsoft ram expander (for faster and smoother performance), titanium backup and link to SD (for uninstalling bloatware and transferring apps to SD card which cannot be done through setting without root. 
So u need root access for your phone to use these apps. 
Rooting your phone can be done through PC by kingoroot, volcano box, etc
And through your phone by kingroot, super su and super user.

I guess this will solve all your problems.. All the best....