The expected increase in particle flux in the high-luminosity phase of the LHC (HL-LHC), with an instantaneous luminosity that can reach $\mathcal{L}=7.5×10^{34}~cm^{−2}~s^{−1}$, will have a significant impact on the pile-up with potentially 200 interactions per bunch crossing. The performances of electrons and photons, as well as those of jets and missing transverse energy, will be strongly degraded in the endcap and the forward regions of the detector, where the granularity of the electromagnetic calorimeter is coarser and the momentum resolution of the Inner Tracker (ITk) is poorer. In order to mitigate the pile-up contamination coming from this high luminosity, a High Granularity Timing Detector (HGTD) is proposed in front of the LAr endcap calorimeters, covering the pseudo-rapidity region between 2.4 and 4.0. The high granularity and the high-precision timing information will improve pile-up reduction. It will also improve the forward object reconstruction, and complement the performance of the updated ITk in the forward region of the ATLAS detector. This leads to an amelioration in the jet and lepton reconstruction performances. The ability of the HGTD detector to improve the pile-up jet rejection and the lepton isolation efficiency within the forward region, in addition to the physics and performance results, is presented.