Porteño Spanish is the dialect of caste-sha-no (castellano) spoken in Buenos Aires, Argentina and the surrounding areas. It is categorized by an Italian-like pronunciation and colorful Lunfardo slang.Browse Slang Dictionary
In general Spanish you can say something is "pesado" (heavy) if it's boring or annoying. "Plomo" is comes from that.
The literal meaning of plomo is "lead", which is very heavy or "pesado".