diff --git a/spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/source/ConfigurationPropertySourcesTests.java b/spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/source/ConfigurationPropertySourcesTests.java
index 52e9f9be3b..f7e05e2630 100644
--- a/spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/source/ConfigurationPropertySourcesTests.java
+++ b/spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/source/ConfigurationPropertySourcesTests.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -19,9 +19,13 @@ package org.springframework.boot.context.properties.source;
 import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.Map;
+import java.util.concurrent.TimeUnit;
 
+import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
 
+import org.springframework.boot.origin.Origin;
+import org.springframework.boot.origin.OriginLookup;
 import org.springframework.core.env.ConfigurableEnvironment;
 import org.springframework.core.env.Environment;
 import org.springframework.core.env.MapPropertySource;
@@ -121,4 +125,71 @@ class ConfigurationPropertySourcesTests {
 		assertThat(configurationSources.iterator()).toIterable().hasSize(5);
 	}
 
+	@Test // gh-20625
+	void environmentPropertyAccessWhenImmutableShouldBePerformant() {
+		testPropertySourcePerformance(true, 1000);
+	}
+
+	@Test // gh-20625
+	@Disabled("for manual testing")
+	void environmentPropertyAccessWhenMutableShouldBeTolerable() {
+		testPropertySourcePerformance(false, 5000);
+	}
+
+	private void testPropertySourcePerformance(boolean immutable, int maxTime) {
+		StandardEnvironment environment = createPerformanceTestEnvironment(immutable);
+		testPropertySourcePerformance(environment, maxTime);
+	}
+
+	private StandardEnvironment createPerformanceTestEnvironment(boolean immutable) {
+		StandardEnvironment environment = new StandardEnvironment();
+		MutablePropertySources propertySources = environment.getPropertySources();
+		for (int i = 0; i < 100; i++) {
+			propertySources.addLast(new TestPropertySource(i, immutable));
+		}
+		ConfigurationPropertySources.attach(environment);
+		return environment;
+	}
+
+	private void testPropertySourcePerformance(StandardEnvironment environment, int maxTime) {
+		long start = System.nanoTime();
+		for (int i = 0; i < 1000; i++) {
+			environment.getProperty("missing" + i);
+		}
+		long total = TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - start);
+		assertThat(environment.getProperty("test-10-property-80")).isEqualTo("test-10-property-80-value");
+		assertThat(total).isLessThan(maxTime);
+	}
+
+	static class TestPropertySource extends MapPropertySource implements OriginLookup<String> {
+
+		private final boolean immutable;
+
+		TestPropertySource(int index, boolean immutable) {
+			super("test-" + index, createProperties(index));
+			this.immutable = immutable;
+		}
+
+		private static Map<String, Object> createProperties(int index) {
+			Map<String, Object> map = new LinkedHashMap<String, Object>();
+			for (int i = 0; i < 1000; i++) {
+				String name = "test-" + index + "-property-" + i;
+				String value = name + "-value";
+				map.put(name, value);
+			}
+			return map;
+		}
+
+		@Override
+		public Origin getOrigin(String key) {
+			return null;
+		}
+
+		@Override
+		public boolean isImmutable() {
+			return this.immutable;
+		}
+
+	}
+
 }