diff --git a/server/src/main/java/org/elasticsearch/index/MergeSchedulerConfig.java b/server/src/main/java/org/elasticsearch/index/MergeSchedulerConfig.java
index ff6987d7d7ff1..709d98a380002 100644
--- a/server/src/main/java/org/elasticsearch/index/MergeSchedulerConfig.java
+++ b/server/src/main/java/org/elasticsearch/index/MergeSchedulerConfig.java
@@ -15,6 +15,10 @@
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.util.concurrent.EsExecutors;
 
+import static org.elasticsearch.common.util.concurrent.EsExecutors.allocatedProcessors;
+import static org.elasticsearch.threadpool.ThreadPool.halfAllocatedProcessorsMaxFive;
+import static org.elasticsearch.threadpool.ThreadPool.twiceAllocatedProcessors;
+
 /**
  * The merge scheduler (<code>ConcurrentMergeScheduler</code>) controls the execution of
  * merge operations once they are needed (according to the merge policy).  Merges
@@ -27,7 +31,7 @@
  * <li> <code>index.merge.scheduler.max_thread_count</code>:
  *
  *     The maximum number of threads that may be merging at once. Defaults to
- *     <code>Math.max(1, Math.min(4, {@link EsExecutors#allocatedProcessors(Settings)} / 2))</code>
+ *     <code>Math.max(1, Math.min(5, {@link EsExecutors#allocatedProcessors(Settings)} / 2))</code>
  *     which works well for a good solid-state-disk (SSD).  If your index is on
  *     spinning platter drives instead, decrease this to 1.
  *
@@ -45,14 +49,14 @@ public final class MergeSchedulerConfig {
 
     public static final Setting<Integer> MAX_THREAD_COUNT_SETTING = new Setting<>(
         "index.merge.scheduler.max_thread_count",
-        (s) -> Integer.toString(Math.max(1, Math.min(4, EsExecutors.allocatedProcessors(s) / 2))),
+        (s) -> Integer.toString(halfAllocatedProcessorsMaxFive(allocatedProcessors(s))),
         (s) -> Setting.parseInt(s, 1, "index.merge.scheduler.max_thread_count"),
         Property.Dynamic,
         Property.IndexScope
     );
     public static final Setting<Integer> MAX_MERGE_COUNT_SETTING = new Setting<>(
         "index.merge.scheduler.max_merge_count",
-        (s) -> Integer.toString(MAX_THREAD_COUNT_SETTING.get(s) + 5),
+        (s) -> Integer.toString(MAX_THREAD_COUNT_SETTING.get(s) + twiceAllocatedProcessors(allocatedProcessors(s))),
         (s) -> Setting.parseInt(s, 1, "index.merge.scheduler.max_merge_count"),
         Property.Dynamic,
         Property.IndexScope
diff --git a/server/src/main/java/org/elasticsearch/index/engine/ThreadPoolMergeExecutorService.java b/server/src/main/java/org/elasticsearch/index/engine/ThreadPoolMergeExecutorService.java
index 5217edb5490dc..33749d3d4a47a 100644
--- a/server/src/main/java/org/elasticsearch/index/engine/ThreadPoolMergeExecutorService.java
+++ b/server/src/main/java/org/elasticsearch/index/engine/ThreadPoolMergeExecutorService.java
@@ -33,7 +33,7 @@ public class ThreadPoolMergeExecutorService {
     /**
      * Floor for IO write rate limit of individual merge tasks (we will never go any lower than this)
      */
-    static final ByteSizeValue MIN_IO_RATE = ByteSizeValue.ofMb(5L);
+    static final ByteSizeValue MIN_IO_RATE = ByteSizeValue.ofMb(10L);
     /**
      * Ceiling for IO write rate limit of individual merge tasks (we will never go any higher than this)
      */
@@ -41,7 +41,7 @@ public class ThreadPoolMergeExecutorService {
     /**
      * Initial value for IO write rate limit of individual merge tasks when doAutoIOThrottle is true
      */
-    static final ByteSizeValue START_IO_RATE = ByteSizeValue.ofMb(20L);
+    static final ByteSizeValue START_IO_RATE = ByteSizeValue.ofMb(40L);
     /**
      * Total number of submitted merge tasks that support IO auto throttling and that have not yet been run (or aborted).
      * This includes merge tasks that are currently running and that are backlogged (by their respective merge schedulers).
diff --git a/server/src/main/java/org/elasticsearch/index/engine/ThreadPoolMergeScheduler.java b/server/src/main/java/org/elasticsearch/index/engine/ThreadPoolMergeScheduler.java
index 8cfdc59268365..7d0a5adf6ef2d 100644
--- a/server/src/main/java/org/elasticsearch/index/engine/ThreadPoolMergeScheduler.java
+++ b/server/src/main/java/org/elasticsearch/index/engine/ThreadPoolMergeScheduler.java
@@ -50,7 +50,7 @@ public class ThreadPoolMergeScheduler extends MergeScheduler implements Elastics
     );
     private final ShardId shardId;
     private final MergeSchedulerConfig config;
-    private final Logger logger;
+    protected final Logger logger;
     private final MergeTracking mergeTracking;
     private final ThreadPoolMergeExecutorService threadPoolMergeExecutorService;
     private final PriorityQueue<MergeTask> backloggedMergeTasks = new PriorityQueue<>(
diff --git a/server/src/main/java/org/elasticsearch/threadpool/DefaultBuiltInExecutorBuilders.java b/server/src/main/java/org/elasticsearch/threadpool/DefaultBuiltInExecutorBuilders.java
index b8dddc20cc51d..e2d14be6db91f 100644
--- a/server/src/main/java/org/elasticsearch/threadpool/DefaultBuiltInExecutorBuilders.java
+++ b/server/src/main/java/org/elasticsearch/threadpool/DefaultBuiltInExecutorBuilders.java
@@ -21,6 +21,7 @@
 
 import static java.util.Collections.unmodifiableMap;
 import static org.elasticsearch.threadpool.ThreadPool.WRITE_THREAD_POOLS_EWMA_ALPHA_SETTING;
+import static org.elasticsearch.threadpool.ThreadPool.halfAllocatedProcessors;
 import static org.elasticsearch.threadpool.ThreadPool.searchAutoscalingEWMA;
 
 public class DefaultBuiltInExecutorBuilders implements BuiltInExecutorBuilders {
@@ -145,7 +146,13 @@ public Map<String, ExecutorBuilder> getBuilders(Settings settings, int allocated
         if (ThreadPoolMergeScheduler.USE_THREAD_POOL_MERGE_SCHEDULER_SETTING.get(settings)) {
             result.put(
                 ThreadPool.Names.MERGE,
-                new ScalingExecutorBuilder(ThreadPool.Names.MERGE, 1, allocatedProcessors, TimeValue.timeValueMinutes(5), true)
+                new ScalingExecutorBuilder(
+                    ThreadPool.Names.MERGE,
+                    1,
+                    halfAllocatedProcessors(allocatedProcessors),
+                    TimeValue.timeValueMinutes(5),
+                    true
+                )
             );
         }
         result.put(
diff --git a/server/src/main/java/org/elasticsearch/threadpool/ThreadPool.java b/server/src/main/java/org/elasticsearch/threadpool/ThreadPool.java
index 85ee02b6db856..302a5d5c6d235 100644
--- a/server/src/main/java/org/elasticsearch/threadpool/ThreadPool.java
+++ b/server/src/main/java/org/elasticsearch/threadpool/ThreadPool.java
@@ -655,7 +655,7 @@ static int halfAllocatedProcessors(final int allocatedProcessors) {
         return (allocatedProcessors + 1) / 2;
     }
 
-    static int halfAllocatedProcessorsMaxFive(final int allocatedProcessors) {
+    public static int halfAllocatedProcessorsMaxFive(final int allocatedProcessors) {
         return boundedBy(halfAllocatedProcessors(allocatedProcessors), 1, 5);
     }
 
@@ -663,7 +663,7 @@ static int halfAllocatedProcessorsMaxTen(final int allocatedProcessors) {
         return boundedBy(halfAllocatedProcessors(allocatedProcessors), 1, 10);
     }
 
-    static int twiceAllocatedProcessors(final int allocatedProcessors) {
+    public static int twiceAllocatedProcessors(final int allocatedProcessors) {
         return boundedBy(2 * allocatedProcessors, 2, Integer.MAX_VALUE);
     }