What is the benefit of Vulcan or Firebird 3?

The main benefit will be a multithreaded engine that can use multiple CPUs effectively. Current versions of Firebird have problems:

SuperServer does not work on multiple CPUs at all. You have to set CPUAffinity in firebird.conf to lock it to a single processor. This is fine when load is not big and your machine acts as something else as well (web server for example), so other applications can use the other CPU. However, it does not do SMP work.

Classic works fine with SMP, but doesn't scale very well. The main problem are locks which are delivered between processes using signals. If you have a lot of connected users (300 for example) on Quad CPU machine, it would mean that all 300 processes need to get lock info from those 4 processes using the database. This communication clutter creates a lot of traffic and load. A proper multithreaded engine that can work on multiple CPUs would solve this, as the communication would be done within a single process without hogging the operating system facilities.