Resolve "Migrate code to OTB 8.x"

Closes #5 (closed)

include/otbSARDEMProjectionImageFilter2.hxx

@@ -333,7 +333,21 @@ SARDEMProjectionImageFilter2< TImageIn, TImageOut >
                 double h; // Elevation from earth geoid
 
 #if OTB_VERSION_MAJOR >= 8
-                // h = demHandler.GetGeoidHeight(demLatLonPoint);
+                // TODO: GetGeoidHeight has very bad performances (*): GDAL
+                // isn't meant to be used for one coordinate at the time.
+                // Instead: have a multi-pass algorithm
+                // 1. get in a SINGLE call the geoid height for all points in
+                //    the buffer
+                // 2. Compute the rest
+                //
+                // (*)
+                // - 96.3% of execution time spent in ThreadedGenerateData
+                //   - 61.3% in GetGeoidHeight
+                //     - 31.3% GDALRasterBand::RasterIO
+                //       - 11.7% in mutex loc+unlock
+                //     - 29.2% OCRCoordinateTransform
+                //       - 20.2% in get_pid
+                //
                 h = GetGeoidHeight(demHandlerTLS, demLatLonPoint);
 #else
                 if (m_geoidEmg96)