diff --git a/daffodil-core/src/main/scala/org/apache/daffodil/core/compiler/Compiler.scala b/daffodil-core/src/main/scala/org/apache/daffodil/core/compiler/Compiler.scala
index 1984c8968d..78db91234d 100644
--- a/daffodil-core/src/main/scala/org/apache/daffodil/core/compiler/Compiler.scala
+++ b/daffodil-core/src/main/scala/org/apache/daffodil/core/compiler/Compiler.scala
@@ -144,7 +144,7 @@ final class ProcessorFactory private (
     codeGenerator
   }
 
-  override def isError: Boolean = sset.isError
+  override lazy val isError: Boolean = sset.isError
 
   def withDistinguishedRootNode(name: String, namespace: String): ProcessorFactory = {
     Assert.usage(name ne null)
@@ -378,8 +378,13 @@ class Compiler private (
         tunables
       )
     }
-    pf.isError // call here to drive compilation, so that compilation at least at the level of the PF
-    // is over when we return the pf.
+    // It is tempting to call pf.isError here to drive compilation to completion before we
+    // return the pf to the caller.
+    // However, this slows down TDML-based testing in Daffodil substantially by moving
+    // the entire isError checking pass inside the synchronized block of the Daffodil
+    // schema compiler. This results in reduced concurrency which substantially slows
+    // the daffodil test suite.
+    // pf.isError // don't call this here. Call it outside the synchronized block.
     pf
   }
 
@@ -410,9 +415,16 @@ object Compiler {
     optRootName: Option[String],
     optRootNamespace: Option[String]
   ): ProcessorFactory = {
-    synchronized {
+    val pf = synchronized {
       c.compileSourceInternal(schemaSource, optRootName, optRootNamespace)
     }
+    // Force all compilation to complete. Called here outside of synchronized block on purpose
+    // to avoid over-serializing things (which would slow down large test suites like Daffodil's test suite.)
+    // Note that this requires that the shared data structures which require Daffodil schema compilation to
+    // be serialized do *not* include the data structures being modified during isError processing (which is
+    // lots of OOLAG evaluations).
+    pf.isError
+    pf
   }
 
 }
diff --git a/daffodil-core/src/main/scala/org/apache/daffodil/core/dsom/SchemaSet.scala b/daffodil-core/src/main/scala/org/apache/daffodil/core/dsom/SchemaSet.scala
index 92d7670f6d..30a06ea47c 100644
--- a/daffodil-core/src/main/scala/org/apache/daffodil/core/dsom/SchemaSet.scala
+++ b/daffodil-core/src/main/scala/org/apache/daffodil/core/dsom/SchemaSet.scala
@@ -683,7 +683,7 @@ final class SchemaSet private (
    * and finally the AST objects are checked for errors, which recursively
    * demands that all other aspects of compilation occur.
    */
-  override def isError: Boolean = {
+  override lazy val isError: Boolean = {
     if (!isValid) true
     else if (
       // use keepGoing so we can capture errors and
@@ -694,6 +694,15 @@ final class SchemaSet private (
       }
     ) true
     else {
+      // we must check for errors here via the super method, as that iterates over
+      // all the objects evaluating them for any errors in their required evaluations.
+      // This has to be after everything else that could report an error here (on some
+      // other object) has been done.
+      //
+      // That is to say, if we called super.isError at the top of this method that would
+      // be incorrect since isValid and areComponentsConstructed above might cause errors
+      // to be recorded or objects created that this call to super.isError would then not
+      // take into account.
       val hasErrors = super.isError
       if (!hasErrors) {
         // must be called after compilation is done
diff --git a/daffodil-lib/src/main/scala/org/apache/daffodil/lib/oolag/OOLAG.scala b/daffodil-lib/src/main/scala/org/apache/daffodil/lib/oolag/OOLAG.scala
index 9bf64780f5..31856bff71 100644
--- a/daffodil-lib/src/main/scala/org/apache/daffodil/lib/oolag/OOLAG.scala
+++ b/daffodil-lib/src/main/scala/org/apache/daffodil/lib/oolag/OOLAG.scala
@@ -478,16 +478,13 @@ object OOLAG {
     }
 
     /**
-     * Currently we depend on being able to evaluate these
-     * repeatedly, and get different answers.
-     *
-     * because it forces evaluation of all the requiredEvaluationsAlways(...)
+     * Forces evaluation of all the requiredEvaluationsAlways(...)
      * or requiredEvaluationsIfActivated(...)
      * on all objects first, but that is only for the objects
      * that have been created and activated at the time this is called.
      */
-
-    def isError: Boolean = {
+    def isError: Boolean = isErrorOnce
+    private lazy val isErrorOnce: Boolean = {
       oolagRoot.checkErrors
       val errorCount = oolagRoot.errors.size
       errorCount > 0