I'm not exactly sure what is happening during the "Forging your Library..." step of startup, but it becomes longer the more pack instances you have, and how big they are. I believe that this is a holdover from the twitch launcher.
Something I noticed was that there is a file that is being written to during this phase in "%appdata%/Local/Overwolf/Curse/GameInstances/" called "MinecraftGameInstances.json.tmp" with many IO calls that I believe is unnecessary, again , I don't know what is going on here. If you can say I'd like to knwo for certain, but my guess is that it's sanning everyfile in the Minecraft Instances folder and adding it to the file as it goes.
My suggestion is to have an index file of the different MC instances, and have a seperate file with that information for each instance instead of only one large file. Also, only scan the Instance forlders as nessessary instead of during start up.