In general the rationale for putting the IP Blocklist early in the chain is that it can search through a large list of blocked addresses without using very much processing power. It doesn’t have to analyse the packet; it just extracts the IP address and looks it up a a series of hash tables.
By putting this early IPFire can avoid processing these packets in later stages, for example in the IPS which can do the same job, but has to analyse the packet in more detail.