Emily

I was facing the same issue with two different modpacks of beta 1.7.3, modpacks that both use jar mods given the age of that game version. Both these instances were...

It's the launcher.

There's also this GitHub repo on the subject of JVM argument tuning for Minecraft, which may contain some additional useful info: https://github.com/brucethemoose/Minecraft-Performance-Flags-Benchmarks

I've experienced this same problem before back in the MultiMC and PolyMC days, specifically with Microsoft accounts under Windows 10. Connectivity checks seem fine under Linux though as I've never...