Your intuition to check the run log and .err file is correct. Some routines may take a while to converge because of bad inputs, and the run log and .err file will usually show which zone and components are responsible. Unfortunately there isn't a way to directly determine which routines are causing a slow down. That is part of a current DOE BTO funded project EnergyPlus 10X to speed-up simulation run time by a factor of 10. Likely through refactoring the surface/zone heat balance routines, HVAC loop simulation, and parallelization.

Until that work is complete, here are some things known to cause high run times (an incomplete list):

• Lots of zones
• Lots of surfaces
• Plenums that connect to lots of surfaces / zones
• Stratified water heaters
• Individual air loop / plant loops / zone equipment for each zone in a multi-zone model
• Lots of shading and high resolution shading/daylighting calculations
• 20+ design days