https://www.overclock3d.net/news/software/monster_hunter_world_iceborne_s_latest_patch_fixes_the_game_s_major_launch_issues/1 Monster Hunter World version 10.12.01 has been released, bringing with it fixes for the major PC issues that arrived with the PC release of the game's Iceborne expansion. These fixes include improvements to CPU utilisation during gameplay, which prevents some CPU-related performance issues, and a fix for the game's save game corruption issue, which prevented save games from before October 30th 2018 from being converted over to the game's new Iceborne compatible save system.
Iceborne's CPU related fixes will deliver performance improvements that are largely system-dependent, with improvements varying between hardware manufacturers and system configurations. One thing to note is that users of pre-October 30th 2018 savegames will still receive an error message, as those files were created before Monster Hunter World received improved PC controls. As such, these save files will be corrupted and lose their customised control setups. Thankfully, Capcom's fix prevents other save data from corrupting, which means that players with affected save files will be able to play Iceborne. The only thing that these players will need to do is rebind their custom control options.
PC players of Monster Hunter World should receive the game's latest update the next time they log into Steam.