Jenkins sends validation requests each time a configuration page is opened
in case the field is empty. In case of 'Use SCM from another project' even
if the option wasn't chosen (let's say Git was enabled or None), Jenkins
anyway will send validation request.

In the doCheckProjectName() if the Project isn't found, then similar projects
are looked up. And if there are a lot of jobs (let's say 5000), then this is
an additional load.