Carbocation are the substances that bear a positive charge. Carbocation are electron deficient substances therefore a Carbocation is stabilized by electron donating groups like Alkyl ,NH2,O- etc.

Carbocation are destabilized by electron withdrawing groups like NO2, aldehyde etc.

If nitrobenzene ring contain a positive charge then as NO2 is electron withdrawing group it will destabilize the charge, so if we compare stability of positive charge on nitrobenzene  with benzene it is less stable in nitrobenzene and hence electrophilic substitution reactions on benzene are fast as compared to nitrobenze.