Performance woes are another major issue being brought up.
Monster Hunter World: Iceborne

Despite Monster Hunter World: Iceborne being released just yesterday for the PC, it has already gotten eviscerated in the reviews on Steam. They seem to have a good reason for it though: Since the release of Iceborne, multiple reports have been made about save files being completely deleted.

Users have come en masse to complain about how their old saves have gone missing, apparently overwritten by a "clean slate" Iceborne save. This has been especially true for those that have not played Monster Hunter for a little while. Sadly, there is no real clear cutoff date that would signify whether or not your save files would be safe or not.

You will know immediately upon starting the game if your save is gone. During the startup process, Monster Hunter World does a check for save files. During this process, it will say that there was "no compatible save found." If you continue from this, the game will then create a new save file and thus overwrite your old data. Those with cloud saves enabled through Steam will then have this new save file uploaded and overwrite their old data upon exiting the game. At that point, there is no way to recover anything.

If you do plan on picking up Iceborne, or if you haven't launched the game yet, you may want to make a backup of your save files before launching the game. You should probably do this regardless of whether or not you played the game recently, just to be safe.

Many users have thought that this issue had something to do with using out of date mods. However, that idea was quickly put to rest after many others encountered the issue who claim to have never used mods. It seems to be that the underlying issue is that there was an update that came out after Monster Hunter World's initial release that changed the format of save data. If you did not play after this update, your save file didn't get updated to a format that is apparently needed for Iceborne. The problem is, nobody really knows when this update happened.

Fans are quite upset by this, as they should be. Unfortunately, it does get worse.

Multiple users, even those without the Iceborne expansion, have reported significant performance issues following the latest update that coincided with Iceborne's launch. Users with high-end gaming machines are reporting framerates that are fractions of what they were getting just prior to the update coming out. In addition, there have been an increased number of crashes being reported as a result of this update. The Steam forums are just one place the issues have been reported. There are also numerous complaints found on Reddit and message boards across the Internet.

A common complaint seems to be that the CPU usage has skyrocketed compared to what it was before. The culprit, at least to what some have discovered, seems to be a new anti-cheat that Capcom has added to the game.

Speaking of Capcom, they do seem to be aware of these issues as they put up a notice of them very shortly after Iceborne was released.

Performance issue occurring under certain conditions

Issue Summary
Thursday, Jan. 9th UTC
We are currently investigating an issue that was brought to our attention with regards to the VU10.12.00 Update that was released earlier today.

Details of the issue
Some users have been experiencing significantly reduced frame rate under certain configuration and environment after applying the latest VU10.12.00 Update.

Please rest assured we're actively investigating the matter at hand and will provide an update as soon as we have more information.
There is a potential workaround for some of the issues the game has been having since yesterday. Some users suggest adding the Monster Hunter directory to your anti-virus exclusions, especially if you're using Windows Security. This may help with some memory leak issues.

Another suggestion to help bring back performance to what it was prior to the update is to do the following:
  1. Start the game
  2. Set every graphical setting to the absolute minimum
  3. Restart the game
  4. Set the graphic settings back to what you had them set to before
  5. Restart the game
Obviously with any user suggested fix, your mileage may vary on how well it works for you.

The best advice we can give to you right now is to backup your old saves at a minimum. If you can, maybe just hold off on attempting to play until Capcom gets some updates out.