diff --git a/internal/util-complete/src/main/scala/sbt/internal/util/complete/Parsers.scala b/internal/util-complete/src/main/scala/sbt/internal/util/complete/Parsers.scala
index b04b61127..c148095ec 100644
--- a/internal/util-complete/src/main/scala/sbt/internal/util/complete/Parsers.scala
+++ b/internal/util-complete/src/main/scala/sbt/internal/util/complete/Parsers.scala
@@ -12,15 +12,17 @@ import Parser._
 import java.io.File
 import java.net.URI
 import java.lang.Character.{
-  getType,
-  MATH_SYMBOL,
-  OTHER_SYMBOL,
+  CURRENCY_SYMBOL,
   DASH_PUNCTUATION,
-  OTHER_PUNCTUATION,
+  MATH_SYMBOL,
   MODIFIER_SYMBOL,
-  CURRENCY_SYMBOL
+  OTHER_PUNCTUATION,
+  OTHER_SYMBOL,
+  getType
 }
 
+import scala.annotation.tailrec
+
 /** Provides standard implementations of commonly useful [[Parser]]s. */
 trait Parsers {
 
@@ -313,6 +315,16 @@ object DefaultParsers extends Parsers with ParserMain {
     apply(p)(s).resultEmpty.isValid
 
   /** Returns `true` if `s` parses successfully according to [[ID]].*/
-  def validID(s: String): Boolean = matches(ID, s)
+  def validID(s: String): Boolean = {
+    // Handwritten version of `matches(ID, s)` because validID turned up in profiling.
+    def isIdChar(c: Char): Boolean = Character.isLetterOrDigit(c) || (c == '_')
+    @tailrec def isRestIdChar(cur: Int, s: String, length: Int): Boolean =
+      if (cur < length)
+        isIdChar(s.charAt(cur)) && isRestIdChar(cur + 1, s, length)
+      else
+        true
+
+    !s.isEmpty && Character.isLetter(s.charAt(0)) && isRestIdChar(1, s, s.length)
+  }
 
 }
diff --git a/internal/util-complete/src/test/scala/DefaultParsersSpec.scala b/internal/util-complete/src/test/scala/DefaultParsersSpec.scala
new file mode 100644
index 000000000..ce3a6b2e9
--- /dev/null
+++ b/internal/util-complete/src/test/scala/DefaultParsersSpec.scala
@@ -0,0 +1,17 @@
+/*
+ * sbt
+ * Copyright 2011 - 2017, Lightbend, Inc.
+ * Copyright 2008 - 2010, Mark Harrah
+ * Licensed under BSD-3-Clause license (see LICENSE)
+ */
+
+package sbt.internal.util
+package complete
+
+import org.scalacheck._, Prop._
+
+object DefaultParsersSpec extends Properties("DefaultParsers") {
+  import DefaultParsers._
+
+  property("validID == matches(ID, s)") = forAll((s: String) => validID(s) == matches(ID, s))
+}