We've improved the hit testing speed a bit, now it could work 5-6 times faster in large diagrams. If anyone is interested in trying that, here is the updated dll:

https://mindfusion.org/_temp/FlowChart.NET.zip

It's a pre-release build of version 4.0.4 and should be pretty stable. Let us know if hit-testing does not work as expected - this includes selection of the correct item upon click, showing auto-handles or auto-anchors when the mouse hovers over an item, etc.