>>WebDev, for example, is website targeted so it splits almost every event into a server-side-code and client-side-code editor since when you, for example, press a button on a website you can choose whether some code needs to execute in the client browser or whether some code needs to execute on the server. This is made very easy because you put a button on a form (web page), open the code editor for it, and it shows you the two code options for the click event; client or server (or both).
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>What about WebDev's performance? I just looked up sample sites and these are not seem so much good. Is it just about their design or WebDev? What's WebDev's technology? Is it about Isapi or something other?

How will you measure the performance of a website (written in anything) by just visiting it over the internet? A web development tool's performance ability needs to be tested on the server, under load conditions, and take into account server configuration, bandwidth, etc. So just visiting a website I don't think is sufficient. I have spoken to WinDev developers who are quite happy with WebDev as a website development tool. But they say if you use the HyperFile built in database and your connections start to exceed 30 or 40 concurrently then you should consider a more powerful 3rd party database like Postgress.

NB: I am not a WinDev/WebDev expert