In old versions of Forge the modloading process was almost exactly as it is now, with the main difference being that Forge now displays what step it is at in the debugging process. The init phase consists of two main parts for each mod, pre-initialization and initialization. There is no way to disable/skip these because without them most mods wouldn't work, the game would likely crash, or if you are lucky you just get hit by extremely heavy lag once you start your world and all the mods load then.
Pre-initialization is the phase where mods overwrite or add code to classes from other mods or the vanilla game code. This is used (sometimes) to add debugging code or (more often) to add compatibility fixes and extra features so that all the loaded mods can work together rather than conflict with each other. The setting up of preinit is actually rather fast, the slow part comes when other mods are loaded and the preinit scans all relevant classes, changing what needs to be changed for the mod to work.
Initialization is the bigger, bulkier phase where all the mods are actually loaded. This is where all the modifications the preinit phase set up are actually executed and injected into code from other mods during their loading. After the mods are fully loaded they do their background setup, often logging individual steps for debugging phrposes.
