diff --git a/core/src/main/scala/org/apache/spark/internal/config/ConfigEntry.scala b/core/src/main/scala/org/apache/spark/internal/config/ConfigEntry.scala
index a295ef06a6376..17d3329e6b494 100644
--- a/core/src/main/scala/org/apache/spark/internal/config/ConfigEntry.scala
+++ b/core/src/main/scala/org/apache/spark/internal/config/ConfigEntry.scala
@@ -89,14 +89,14 @@ private[spark] abstract class ConfigEntry[T] (
   def defaultValueString: String
 
   protected def readString(reader: ConfigReader): Option[String] = {
-    val values = Seq(
-      prependedKey.flatMap(reader.get(_)),
-      alternatives.foldLeft(reader.get(key))((res, nextKey) => res.orElse(reader.get(nextKey)))
-    ).flatten
-    if (values.nonEmpty) {
-      Some(values.mkString(prependSeparator))
-    } else {
-      None
+    // SPARK-48678: performance optimization: this code could be expressed more succinctly
+    // using flatten and mkString, but doing so adds lots of Scala collections perf. overhead.
+    val maybePrependedValue: Option[String] = prependedKey.flatMap(reader.get)
+    val maybeValue: Option[String] = alternatives
+      .foldLeft(reader.get(key))((res, nextKey) => res.orElse(reader.get(nextKey)))
+    (maybePrependedValue, maybeValue) match {
+      case (Some(prependedValue), Some(value)) => Some(s"$prependedValue$prependSeparator$value")
+      case _ => maybeValue.orElse(maybePrependedValue)
     }
   }
 
diff --git a/core/src/main/scala/org/apache/spark/internal/config/ConfigReader.scala b/core/src/main/scala/org/apache/spark/internal/config/ConfigReader.scala
index c1ab22150d024..8824d196489a8 100644
--- a/core/src/main/scala/org/apache/spark/internal/config/ConfigReader.scala
+++ b/core/src/main/scala/org/apache/spark/internal/config/ConfigReader.scala
@@ -84,7 +84,9 @@ private[spark] class ConfigReader(conf: ConfigProvider) {
   def substitute(input: String): String = substitute(input, Set())
 
   private def substitute(input: String, usedRefs: Set[String]): String = {
-    if (input != null) {
+    // SPARK-48678: performance optimization: skip the costly regex processing
+    // if the string cannot possibly contain a variable reference:
+    if (input != null && input.contains("${")) {
       ConfigReader.REF_RE.replaceAllIn(input, { m =>
         val prefix = m.group(1)
         val name = m.group(2)