I had the crash thing with 2018 when I installed some plugin, de-installed that and all was well again, so maybe check that.

Performance should be the same or maybe even better then 2018 are you sure your machine is OK with running Revit (obvious question, but got to ask).

No idea about the warnings, so far have not seen anything different compared to 2018.

The RVT 2019 family folder is the one that comes with installing revit 2019? If so it seems like Autodesk might have forgotten to update the families. If it is your own family folder, you will have to upgrade all your families to 2019. I have never used an OOTB family, so can not tell you if that is the case.

And I do hope you tested 2019 before you updated all your projects. You can always finish your projects in 2018 (unless they are all updated to 2019).

There was a post in the forum a while ago about a known issue with general slowdown in Revit 2019 if you had the Roombook extension installed - not sure if Autodesk have fixed that yet.

It may be completely unrelated but I only had issues with Revit 2019.1 via Windows 10 updates - update build 1803 in particular - and conflicts with Office 365. Attempting to uninstall Office 365 was a very bad idea and in the end I had to reinstall Windows 10 from scratch and now everything is lovely - although it took 12 attempts :banghead:

We have been recently having this issue with 2019.1 on 360. This occurs at random on both Windows 7 and Windows 10 machines.

Generally ill advised to mix Operating System versions among users contributing to the same project files. Same goes for Revit builds.