Index: trunk/src/org/openstreetmap/josm/data/imagery/WMTSTileSource.java
===================================================================
--- trunk/src/org/openstreetmap/josm/data/imagery/WMTSTileSource.java	(revision 13509)
+++ trunk/src/org/openstreetmap/josm/data/imagery/WMTSTileSource.java	(revision 13510)
@@ -102,4 +102,6 @@
         PATTERN_HEADER,
     };
+
+    private int cachedTileSize = -1;
 
     private static class TileMatrix {
@@ -733,4 +735,7 @@
     @Override
     public int getTileSize() {
+        if (cachedTileSize > 0) {
+            return cachedTileSize;
+        }
         if (tileProjection != null) {
             // no support for non-square tiles (tileHeight != tileWidth)
@@ -738,5 +743,6 @@
             Collection<Layer> projLayers = getLayers(null, tileProjection.toCode());
             if (!projLayers.isEmpty()) {
-                return projLayers.iterator().next().tileMatrixSet.tileMatrix.get(0).tileHeight;
+                cachedTileSize = projLayers.iterator().next().tileMatrixSet.tileMatrix.get(0).tileHeight;
+                return cachedTileSize;
             }
         }