The rapid translocation speed of peptides through graphene nanopores poses a challenge, hindering the accurate sensing of the biomarkers. Employing the functionalized graphene nanopores is at the forefront of reducing the translocation speed. The current work details the translocation of a negatively charged peptide endothelin-1 through a bare multilayer graphene nanopore, a hydrogen-functionalized graphene nanopore, and a hydroxyl-functionalized graphene nanopore by applying electric fields. The hydroxyl-functionalized graphene nanopore significantly reduces the peptide's translocation speed. The time required for the peptide to translocate through the hydroxyl-functionalized graphene nanopore is 2.25 times longer than in the non-functionalized graphene nanopore and 1.25 times longer than in the hydrogen-functionalized graphene nanopore. We critically analyze the factors influencing the reduced translocation speed, including the interactions between the pore and the peptide, the conformational changes of the peptide within the pore, the solvent velocity inside the pore, and the solvent's viscosity near the peptide. The altered solvent velocities within functionalized pores have a minimal role in the speed reduction of peptides. When a constant force is applied to the peptide without any electric field, the hydroxyl-functionalized graphene nanopore delivers the lowest diffusion rate. The persistence time, which serves as a measure of the solvent viscosity near the peptide, is the highest within the hydroxyl-functionalized pore. Finally, we conclude that the Coulombic interactions between the peptide and the pore play a major role in its speed reduction inside the hydroxyl-functionalized graphene nanopore.