PCE Emacs blinking on xml files

It complains regularly No <!DOCTYPE declaration. I don’t care. There is no doctype, it’s just an xml file with elements. How can I turn off the error?

Simply creating x.xml containing


doesn’t show any problem …

I attempted to create a demonstration example, and whenever I delete a certain volume of elements from the file, it stops erroring. I think it’s a time-out on the colorizing process that’s at fault here.