There are a lot of optimizations that we plan to add to this path. The intent of this preview was getting a functional version of `dotnet run app.cs` out the door. Items like startup optimization are going to be coming soon.
There is no timeframe right now. At the moment this is a loose proposal for how to approach union like features in C#. There are a lot more details to dig into here before we'd be ready to move to scheduling and implementation. Also need to decide if we take on all of the union variants proposed or just some and what priority order to approach them.
The design is not going to allow for struct layout to have memory safety issues when tearing is involved. That is a problem we are very well aware of and consider in designs.
What version are you using? Prior to 1.2 there was a nasty perf bug with :hlsearch that caused it to be sluggish with 1) very large files or 2) a good number of files opened. The bug is mostly fixed in 1.2 but there are still a couple of corner cases. Those are addressed in 1.3 which will be released sometime next week. A beta drop is available here
What version of VsVim are you using? There was a bug with this exact behavior in 1.1 (and possibly 1.1.1). I got it fixed for the 1.2 release which is the current version in the Visual Studio gallery.