Hi,
You will need to rebuild your custom project after every change (even changes in .aspx or .ascx files) because it is the post build event which must copy your files up to run in the context of mojoPortal.Web project.
However, you can work much faster by not using the debugger. The debugger really slows things down but you only need to use the debugger when you actually want to step through the code as it executes. Usually I only do that when there is something not working right and I'm trying to find out exactly what is happening.
Instead of launching the debugger, assuming that you have rebuilt the entire solution at least once, you can just right click the Default.aspx file in the root of mojoPortal.Web project in Solution Explorer and choose "View In Browser", it will launch the VS Web server without loading the debugger and it will work much faster. Then when you make a change in your project, you still have to rebuild your project (not the whole solution), and then just refresh the page in the browser.
Any time something doesn't work as expected you can then use the debugger and step through the code to see what is happening.
Hope that helps,
Joe