Index: source/graphics/MapGenerator.h
===================================================================
--- source/graphics/MapGenerator.h
+++ source/graphics/MapGenerator.h
@@ -23,6 +23,7 @@
 #include "ps/TemplateLoader.h"
 #include "scriptinterface/StructuredClone.h"
 
+#include <atomic>
 #include <boost/random/linear_congruential.hpp>
 #include <mutex>
 #include <set>
@@ -57,7 +58,7 @@
 	 *
 	 * @return Progress percentage 1-100 if active, 0 when finished, or -1 on error
 	 */
-	int GetProgress();
+	int GetProgress() const;
 
 	/**
 	 * Get random map data, according to this format:
@@ -100,7 +101,7 @@
 	 *
 	 * @return Progress percentage 1-100 if active, 0 when finished, or -1 on error
 	 */
-	int GetProgress();
+	int GetProgress() const;
 
 	/**
 	 * Get random map data, according to this format:
@@ -199,7 +200,7 @@
 	/**
 	 * Current map generation progress.
 	 */
-	int m_Progress;
+	std::atomic<int> m_Progress;
 
 	/**
 	 * Provides the script context.
Index: source/graphics/MapGenerator.cpp
===================================================================
--- source/graphics/MapGenerator.cpp
+++ source/graphics/MapGenerator.cpp
@@ -97,7 +97,6 @@
 		if (!Run() || m_Progress > 0)
 		{
 			// Don't leave progress in an unknown state, if generator failed, set it to -1
-			std::lock_guard<std::mutex> lock(m_WorkerMutex);
 			m_Progress = -1;
 		}
 
@@ -210,9 +209,8 @@
 #undef REGISTER_MAPGEN_FUNC
 #undef REGISTER_MAPGEN_FUNC_NAME
 
-int CMapGeneratorWorker::GetProgress()
+int CMapGeneratorWorker::GetProgress() const
 {
-	std::lock_guard<std::mutex> lock(m_WorkerMutex);
 	return m_Progress;
 }
 
@@ -237,13 +235,15 @@
 
 void CMapGeneratorWorker::SetProgress(int progress)
 {
-	// Copy data
-	std::lock_guard<std::mutex> lock(m_WorkerMutex);
+	const int currentProgress = m_Progress;
+	if (progress < currentProgress)
+	{
+		LOGWARNING("The random map script tried to reduce the loading progress from %d to %d", currentProgress, progress);
+		return;
+	}
 
-	if (progress >= m_Progress)
-		m_Progress = progress;
-	else
-		LOGWARNING("The random map script tried to reduce the loading progress from %d to %d", m_Progress, progress);
+	const int previousProgress = m_Progress.exchange(progress);
+	ENSURE(previousProgress <= progress);
 }
 
 CParamNode CMapGeneratorWorker::GetTemplate(const std::string& templateName)
@@ -413,7 +413,7 @@
 	m_Worker->Initialize(scriptFile, settings);
 }
 
-int CMapGenerator::GetProgress()
+int CMapGenerator::GetProgress() const
 {
 	return m_Worker->GetProgress();
 }
Index: source/graphics/MapReader.cpp
===================================================================
--- source/graphics/MapReader.cpp
+++ source/graphics/MapReader.cpp
@@ -29,7 +29,6 @@
 #include "graphics/TerrainTextureEntry.h"
 #include "graphics/TerrainTextureManager.h"
 #include "lib/timer.h"
-#include "lib/external_libraries/libsdl.h"
 #include "maths/MathUtil.h"
 #include "ps/CLogger.h"
 #include "ps/Loader.h"
@@ -1313,14 +1312,6 @@
 			m_MapData.init(rq.cx, data);
 		}
 	}
-	else
-	{
-		// Still working
-
-		// Sleep for a while, slowing down the rendering thread
-		// to allow more CPU for the map generator thread
-		SDL_Delay(100);
-	}
 
 	// return progress
 	return progress;