Background: The aim of this study was to evaluate inter-observer variability in terms of treatment planning (TP) quality and treatment delivery (TD) efficiency in the setting of IMRT plans and to identify potential optimization objectives that can be implemented in institutional optimization protocols. Materials and Methods: Four different observers generated IMRT plans for 15 patients with prostate cancer. Plans were evaluated in terms of inter-observer variability considering dosimetric objectives regarding TP quality (using planning target volume (PTV) coverage, conformity index (CI), homogeneity index (HI), organs at risk (OAR) dose constraints and remaining volume at risk (RVR) doses) and regarding TD efficiency (using the mean number of segments, the mean values for total MUs, the mean values for maximum beam MUs and the mean TD time). Results: Regarding TP quality, there were no clinically significant differences among observers in terms of PTV coverage, CI, HI, OAR dose constraints and RVR doses. Regarding TD efficiency, there were statistically significant differences among observers in terms of the mean number of segments, the mean values for total MUs, the mean values for maximum beam MUs and the mean TD times. Conclusions: Even for IMRT plans generated according to standardized protocols, TD times significantly differ among planners. The limitation of the number of segments per beam and maximum beam MUs during optimization can lower TD times as well as total MUs and improve TD efficiency. Pre-determined optimization protocols can enable easier transfer of experiences, act as time-savers and result in a more efficient workflow in busy clinics.