I haven't migrated an application like you did (always rewrote the application), but this is what I would do / consider:
- rewrite code that is really slow
- use .NET framework. Write code that can replace external components or wrappers. (Like SerialPort instead of some rs232 class).
- use BackGroundWorker for time consuming actions.
http://msdn.microsoft.com/en-us/library/system.componentmodel.backgroundworker(v=VS.90).aspx[
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- use a splash screen at startup and other visual effects like hourglass, progressbar, clock or anything that shows the user that the application is busy.
- place the assembly in the Global Assembly Cache (GAC). I wouldn't recommend it, but it is something to consider.
http://msdn.microsoft.com/en-us/library/ex0ss12c(v=VS.90).aspx[
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http://social.msdn.microsoft.com/Forums/en-US/clr/thread/7a6d0d3e-24b5-4f0a-ac59-b7bb0e1c4611/[
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- how are you connecting to the database?
- and like cuteband advised: use stored procedures
I hope this gives you some ideas and options.