It sound strange to reply to oneself questions but anyway, I've found
what cause Visual Web Developer to be so slow on loading .rdlc files.

The problem rise when the "Microsoft.ReportViewer.WebForms" namespace
is registered inside the application's web.config file. That happens
when you add a reference in the project to the
Microsoft.ReportViewer.WebForms.dll assembly.

I don't know why that (the slow-down) happens but if one get the
"Microsoft.ReportViewer.WebForms" namespace registration out of the
web.config file, rdlc files are opened in matter of second, if the
registration is present, it takes minutes to VWD to load up the rdlc.
And that doesn't happens just on my computer. I've tested two other
computers and the behaviour is the same on both.

I'm wondering wheter this is a known issue or not?

Now that I know that I'm facing another problem: I need that
registration somewhere because I've some utility code on a class that
works with objects from that namespace.
Where can I register the "Microsoft.ReportViewer.WebForms" namespace
other than the web.config file so that my classes can access it?

Thanks for the help.