Pretty much what it says on the tin. The Inferno Lord's Focus can proc itself with its sparks. The closer you approach 100% crit chance, the more likely this will cause a runaway feedback loop, which causes extreme lag - but not freezing, so I assume it stops once the sparks deal 0 damage. I'm doing a run with Thorium+Calamity+a bunch of other mods which let me easily push crit chance past 100%, but it becomes an issue with the following Thorium-only loadouts as well (among others, I'm sure):

- Unreal S.D.M.G // Assassin Set (Scouter) // Ranger Emblem, Sniper Scope, Recon Scope, Destroyer Emblem, Mask of the Crystal Eye, Inferno Lord's Focus
- Mythical Devil's Claw // Pyromancer Set (Cowl) // Celestial Emblem, Eye of the Golem, Putrid Scent, Destroyer Emblem, Mask of the Crystal Eye, Inferno Lord's Focus
(accessories all Lucky)

Use that loadout on any boss that can survive more than a second and you should see it happen - moreso if you swap out the Celestial Emblem for The Ring, or sip a rage potion. While the focus is an endgame accessory normally, it's available right at the start of Calamity's post-Moon Lord content and, well... just not letting it proc off itself should solve the issue with minimal change to its damage, right? It'd still be a fantastic accessory; it just wouldn't turn your game into a slideshow the moment you decide to use an AOE magic weapon on a worm boss.