In every city, there were/are slums. Paris, Barcelona, Madrid, etc, etc. The majority of Victorian Glasgow were not slum areas. For example, Shawlands, Dennistoun, Partick, Park Circus, Jordanhill, Newlands, Cathcart, Muirend, Pollokshields, Pollokshaws, Hyndland, and of course many more areas!
In my opinion, the majority of Glasgow was not built as a slum.