Windows Workflow underwent a major overhaul for .NET 4.0 and was largely a ground-up rewrite. As part of that rewrite, the workflow designer was also rebuilt; however, it was still fairly slow, and with medium to large workflows, it was more than capable of crashing Visual Studio, taking all your unsaved changes with it and generally annoying anyone who had to use it for long periods of time.
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What is a workflow? Developer Dino Esposito has said, "… a workflow-based solution consists of making complex processes simpler to model and implement…" Microsoft's MSDN provides in-depth information on creating workflows, starting with a conceptual overview available at http://msdn.microsoft.com/en-us/library/dd489465(v=vs.110).aspx.
In Visual Studio 2012 and higher, the workflow designer has been given some additional attention to improve its stability and performance. For the workflow engine itself, Microsoft has added much needed support for state machines...