Hi Alexey,
The main advantage of LFA FRR is the increased convergence time so it’s probably best only to use this on routers where switching over from the primary to backup path really makes a difference, like routers forwarding VoIP traffic where the lower convergence time is important. I wouldn’t just enable this on all routers in your network.
Once it switches over to the backup path, it does run SPF to re-calculate. You can see it in action if you enable debug ip ospf fast-reroute and debug ip ospf spf.