diff --git a/core/src/main/scala/sttp/model/UriInterpolator.scala b/core/src/main/scala/sttp/model/UriInterpolator.scala
index a18463ec..0481e494 100644
--- a/core/src/main/scala/sttp/model/UriInterpolator.scala
+++ b/core/src/main/scala/sttp/model/UriInterpolator.scala
@@ -5,6 +5,7 @@ import sttp.model.internal.{ArrayView, FastCharSet, FastCharMap, ParseUtils, Rfc
 import scala.annotation.tailrec
 import scala.collection.immutable.VectorBuilder
 import scala.collection.mutable.ArrayBuffer
+import scala.reflect.ClassTag
 
 trait UriInterpolator {
   implicit class UriContext(val sc: StringContext) {
@@ -43,6 +44,10 @@ object UriInterpolator {
 
   private val startingUri = Uri(None, None, Uri.EmptyPath, Nil, None)
 
+  // Dynamic class tag resultion uses `synchronized` and can block threads.
+  // Cache the class tag locally to avoid it.
+  private implicit val tokenClassTag: ClassTag[Token] = ClassTag(classOf[Token])
+
   private val builders = List(
     UriBuilder.Scheme,
     UriBuilder.UserInfo,