grass - Solar irradiation calculation using r.sun

I am trying to compute the global solar irradiation for a region and the run times seem to be a bit too high.

I have as inputs a Digital Surface Model with the buildings and a Digital Terrain Model for the surrounding area.

The PROJ_INFO is

name       : Lat/Lon
proj       : ll
datum      : wgs84
ellps      : wgs84

The PROJ_UNITS are

unit       : degree
units      : degrees
meters     : 1.0

The DSM has 8020 Rows, 8782 Columns and 70431640 Total Cells. The resolution of the DSM is 0:00:00.03303.

The DTM has 1080 Rows, 1476 Columns and 1594080 Total Cells with a resolution of 0:00:00.28913.

These are the steps I did for the r.sun calculation

1. 
   

   Set the region to the area covered by the DTM and the resolution to the resolution of the DSM

   
   

   g.region rast=DTM@PERMANENT res=0:00:00.03303



2. 
   

   Overlay the DSM and the DTM to get a high resolution elevation map using r.series

   
   
   

   r.series input=DSM,DTM output=Overlay method=maximum



3. 
   

   From the Overlay map I generate slope and aspect maps

   
   

   r.slope.aspect elevation=Overlay slope=Slope aspect=Aspect



4. 
   

   I run r.sun on using the following command

   
   

   r.sun -s elevin=DSM aspin=Aspect slopein=Slope glob_rad=GlobalRad day=262

The whole process ran on a system with two Intel Xeon l5430 CPU @ 2.66 GHz with 8 GB of RAM and it took 15 hours to finalize 55% of the process.

What am I doing wrong?

Am I missing something, is it normal to run this long?

Help me understand how should I compute solar irradiation starting from two elevation maps, a Digital Surface Model that contains the buildings and a Digital Terrain Model for the area that surrounds the Digital Surface Model.