overloading efficiency

I have read that using overloaded functions is expensive because overloading is implemented by passing dictionaries around. These various sources seem to agree that this in necessary to allow separate compilation. I don't understand why this is... Couldn't the functions be specialized at link time? -- /Times-Bold 40 selectfont/n{moveto}def/m{gsave true charpath clip 72 400 n 300 -4 1{dup 160 300 3 -1 roll 0 360 arc 300 div 1 1 sethsbcolor fill}for grestore 0 -60 rmoveto}def 72 500 n(This message has been)m (brought to you by the)m(letter alpha and the number pi.)m(David Feuer) m(David_Feuer@brown.edu)m showpage