diff --git a/jsonld.php b/jsonld.php
index 3dc15a9..053d008 100644
--- a/jsonld.php
+++ b/jsonld.php
@@ -4695,16 +4695,16 @@ class JsonLdProcessor {
     }
 
     // no term or @vocab match, check for possible CURIEs
+    $iri_len = strlen($iri);
     $choice = null;
     foreach($active_ctx->mappings as $term => $definition) {
-      // skip terms with colons, they can't be prefixes
-      if($definition && $definition->_term_has_colon) {
+      // skip null definitions and terms with colons, they can't be prefixes
+      if($definition === null || $definition->_term_has_colon) {
         continue;
       }
       // skip entries with @ids that are not partial matches
-      if($definition === null ||
-        $definition->{'@id'} === $iri ||
-        strpos($iri, $definition->{'@id'}) !== 0) {
+      if(!($iri_len > $definition->_id_length &&
+        strpos($iri, $definition->{'@id'}) === 0)) {
         continue;
       }
 
@@ -4712,7 +4712,7 @@ class JsonLdProcessor {
       // 1. it has no mapping, OR
       // 2. value is null, which means we're not compacting an @value, AND
       //   the mapping matches the IRI)
-      $curie = $term . ':' . substr($iri, strlen($definition->{'@id'}));
+      $curie = $term . ':' . substr($iri, $definition->_id_length);
       $is_usable_curie = (!property_exists($active_ctx->mappings, $curie) ||
         ($value === null && $active_ctx->mappings->{$curie} &&
         $active_ctx->mappings->{$curie}->{'@id'} === $iri));
@@ -4990,6 +4990,9 @@ class JsonLdProcessor {
       }
     }
 
+    // optimization to store length of @id once for _compactIri
+    $mapping->_id_length = strlen($mapping->{'@id'});
+
     // IRI mapping now defined
     $defined->{$term} = true;