When an external display Mac, unless access is Apple's own monitor "ACD", otherwise commonly encountered fuzzy fonts send virtual problems. Execution command in the terminal:
defaults -currentHost write -globalDomain AppleFontSmoothing -int 2
1 to 3 may be used as the last parameter of the command representing the font rendering smoothing intensity. If you want to restore the default settings:
defaults -currentHost delete -globalDomain AppleFontSmoothing