For details on what's coming in VistaDB Version 6 see:
When can we expect VistaDB6?
We don't have a public date on that; we are presently working through the locking changes so we can release a beta with the new async API, multithreading, and locking.
we tried VistaDB5 a while ago but it was a bit to slow for our case. Now we have an other project for which we have to decide till mid April which db to use. Is it realistic that till than a alpha/beta release will be available?
We're expecting a beta to be available within the next 2-3 weeks. It'll have several but not all of the performance enhancements planned for 6.0. I'd recommend opening a support ticket (email@example.com) with your specifics so we can give you guidance on how to make it faster in general with VistaDB and if the changes for 6.0 are likely to improve those scenarios.