https://www.kitguru.net/gaming/matthew-wilson/monster-hunter-world-iceborne-launches-with-pc-issues-capcom-is-investigating/
 
Currently, Iceborne has a ‘mixed’ reception on Steam, with some of the highest rated reviews pointing out performance issues. In particular, much higher CPU usage seems to be a common problem, causing performance hits in old areas of the game in addition to the brand new area. Beyond that, a lot of players are taking issue with the fact that some of the default keybindings changed with the introduction of Iceborne, forcing players to re-learn certain actions, or dive back into the options menu to fix their settings.
 
A rarer but major issue is the fact that some players have lost their save files. The Iceborne expansion launch came with an update to the Monster Hunter World base game too and some players have encountered an error with save files not being compatible with the latest version of the game. Once you exit the game, that save file is automatically uploaded to Steam Cloud Saves, causing more of a headache.
 
Capcom is aware of these issues at the moment and is investigating but so far, we don’t have a fix. In the meantime,  Nexusmods has Iceborne-ready save files and save transfer tools that may help some users jump in quicker. You may also want to ensure that you have a local backup of your Monster Hunter World save files, or disable Steam Cloud syncing with the game before launching Iceborne for the first time.
 
I am hoping that a game patch will come soon to resolve many issues.