@meisterT -- `checking cached actions` now takes 5-6x longer in my local builds after `6ac6954224b2b74c18d3218dfa299424cbc944fb` was merged, that is what used to take 30 seconds for bazel to determine there was nothing to do in an already fully cached build, it now takes 2 mins 30 seconds There used to be a check against this boolean which I assume was defaulting to `true`:
```
-  private static final AtomicBoolean MULTI_THREADED_DIGEST = new AtomicBoolean(false);
...
-    if (fileSize > MULTI_THREADED_DIGEST_MAX_FILE_SIZE && !MULTI_THREADED_DIGEST.get()) {
+    if (fileSize > MULTI_THREADED_DIGEST_MAX_FILE_SIZE) {
       // We'll have to read file content in order to calculate the digest.
       // We avoid overlapping this process for multiple large files, as
       // seeking back and forth between them will result in an overall loss of
       // throughput.
       digest = getDigestInExclusiveMode(path);
     } else {
-      digest = getDigestInternal(path);
+      digest = path.getDigest();
     }
```
But now it always takes the slow path. I revived the fast `checking cached actions` with this:
```
-    if (fileSize > MULTI_THREADED_DIGEST_MAX_FILE_SIZE) {
+    if (fileSize > MULTI_THREADED_DIGEST_MAX_FILE_SIZE && !true) {
``` Ouch, that's definitely a mistake. I'm OOO this week but feel free to send a PR to fix the behavior.

cc @Wyverald  this is a release blocker Cherrypicks are still open. Should be reopened as it's still blocking 5.0 (needs to be cherry-picked). Cherrypicked.
Drop experimental_multi_threaded_digest
This options defaults to true since a while and should just be removed, likely with the whole `SsdModule`.
src/main/java/com/google/devtools/build/lib/vfs/DigestUtils.java
src/main/java/com/google/devtools/build/lib/vfs/DigestUtils.java
src/test/java/com/google/devtools/build/lib/vfs/DigestUtilsTest.java
diff --git a/src/main/java/com/google/devtools/build/lib/vfs/DigestUtils.java b/src/main/java/com/google/devtools/build/lib/vfs/DigestUtils.java
index e22e9577f5c055..d3841ea8ad0306 100644
--- a/src/main/java/com/google/devtools/build/lib/vfs/DigestUtils.java
+++ b/src/main/java/com/google/devtools/build/lib/vfs/DigestUtils.java
@@ -16,11 +16,8 @@
 import com.github.benmanes.caffeine.cache.Cache;
 import com.github.benmanes.caffeine.cache.Caffeine;
 import com.github.benmanes.caffeine.cache.stats.CacheStats;
-import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.Preconditions;
 import com.google.common.primitives.Longs;
-import com.google.devtools.build.lib.profiler.Profiler;
-import com.google.devtools.build.lib.profiler.ProfilerTask;
 import java.io.IOException;
 
 /**
@@ -41,17 +38,9 @@
  * fail.
  */
 public class DigestUtils {
-  // Object to synchronize on when serializing large file reads.
-  private static final Object DIGEST_LOCK = new Object();
-
   // Typical size for a digest byte array.
   public static final int ESTIMATED_SIZE = 32;
 
-  // Files of this size or less are assumed to be readable in one seek.
-  // (This is the default readahead window on Linux.)
-  @VisibleForTesting // the unittest is in a different package!
-  public static final int MULTI_THREADED_DIGEST_MAX_FILE_SIZE = 128 * 1024;
-
   /**
    * Keys used to cache the values of the digests for files where we don't have fast digests.
    *
@@ -126,19 +115,6 @@ public int hashCode() {
   /** Private constructor to prevent instantiation of utility class. */
   private DigestUtils() {}
 
-  /**
-   * Obtain file's digset using synchronized method, ensuring that system is not overloaded in case
-   * when multiple threads are requesting digest calculations and underlying file system cannot
-   * provide it via extended attribute.
-   */
-  private static byte[] getDigestInExclusiveMode(Path path) throws IOException {
-    long startTime = Profiler.nanoTimeMaybe();
-    synchronized (DIGEST_LOCK) {
-      Profiler.instance().logSimpleTask(startTime, ProfilerTask.WAIT, path.getPathString());
-      return path.getDigest();
-    }
-  }
-
   /**
    * Enables the caching of file digests based on file status data.
    *
@@ -221,16 +197,7 @@ public static byte[] manuallyComputeDigest(Path path, long fileSize) throws IOEx
       }
     }
 
-    // Compute digest from the file contents.
-    if (fileSize > MULTI_THREADED_DIGEST_MAX_FILE_SIZE) {
-      // We'll have to read file content in order to calculate the digest.
-      // We avoid overlapping this process for multiple large files, as
-      // seeking back and forth between them will result in an overall loss of
-      // throughput.
-      digest = getDigestInExclusiveMode(path);
-    } else {
-      digest = path.getDigest();
-    }
+    digest = path.getDigest();
 
     Preconditions.checkNotNull(digest, "Missing digest for %s (size %s)", path, fileSize);
     if (cache != null) {

diff --git a/src/test/java/com/google/devtools/build/lib/vfs/DigestUtilsTest.java b/src/test/java/com/google/devtools/build/lib/vfs/DigestUtilsTest.java
index 7e2d5368cfbc03..b93d2c8244ed20 100644
--- a/src/test/java/com/google/devtools/build/lib/vfs/DigestUtilsTest.java
+++ b/src/test/java/com/google/devtools/build/lib/vfs/DigestUtilsTest.java
@@ -20,7 +20,6 @@
 import com.google.devtools.build.lib.testutil.TestUtils;
 import com.google.devtools.build.lib.vfs.inmemoryfs.InMemoryFileSystem;
 import java.io.IOException;
-import java.util.Arrays;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicInteger;
@@ -93,25 +92,6 @@ protected byte[] getFastDigest(PathFragment path) throws IOException {
      thread2.joinAndAssertState(TestUtils.WAIT_TIMEOUT_MILLISECONDS);
   }
 
-  /**
-   * Ensures that digest calculation is synchronized for files greater than
-   * {@link DigestUtils#MULTI_THREADED_DIGEST_MAX_FILE_SIZE} bytes if the digest is not
-   * available cheaply, so machines with rotating drives don't become unusable.
-   */
-  @Test
-  public void testCalculationConcurrency() throws Exception {
-    int small = DigestUtils.MULTI_THREADED_DIGEST_MAX_FILE_SIZE;
-    int large = DigestUtils.MULTI_THREADED_DIGEST_MAX_FILE_SIZE + 1;
-    for (DigestHashFunction hf :
-        Arrays.asList(DigestHashFunction.SHA256, DigestHashFunction.SHA1)) {
-      assertDigestCalculationConcurrency(true, true, small, small, hf);
-      assertDigestCalculationConcurrency(true, true, large, large, hf);
-      assertDigestCalculationConcurrency(true, false, small, small, hf);
-      assertDigestCalculationConcurrency(true, false, small, large, hf);
-      assertDigestCalculationConcurrency(false, false, large, large, hf);
-    }
-  }
-
   @Test
   public void testCache() throws Exception {
     AtomicInteger getFastDigestCounter = new AtomicInteger(0);
