diff --git a/include/nlohmann/detail/input/lexer.hpp b/include/nlohmann/detail/input/lexer.hpp
index b5cc8d712..5861bd49d 100644
--- a/include/nlohmann/detail/input/lexer.hpp
+++ b/include/nlohmann/detail/input/lexer.hpp
@@ -1705,9 +1705,20 @@ scan_number_done:
         // nothing below calls unget()
         get();
 
-        while (current == ' ' || current == '\t' || current == '\n' || current == '\r')
+        // this is written as an if-guarded do-while (rather than a plain
+        // while loop) because that shape is what lets both GCC and Clang
+        // keep the input adapter's read pointer in a register across
+        // iterations; the equivalent while-loop measurably defeated that
+        // optimization in testing, turning long whitespace runs (e.g. the
+        // indentation of pretty-printed JSON) from a register-only loop
+        // into one that reloads the pointer from memory every character
+        if (current == ' ' || current == '\t' || current == '\n' || current == '\r')
         {
-            get_ignoring_pending_unget();
+            do
+            {
+                get_ignoring_pending_unget();
+            }
+            while (current == ' ' || current == '\t' || current == '\n' || current == '\r');
         }
     }
 
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 5532ce752..2a787ce8a 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -9488,9 +9488,20 @@ scan_number_done:
         // nothing below calls unget()
         get();
 
-        while (current == ' ' || current == '\t' || current == '\n' || current == '\r')
+        // this is written as an if-guarded do-while (rather than a plain
+        // while loop) because that shape is what lets both GCC and Clang
+        // keep the input adapter's read pointer in a register across
+        // iterations; the equivalent while-loop measurably defeated that
+        // optimization in testing, turning long whitespace runs (e.g. the
+        // indentation of pretty-printed JSON) from a register-only loop
+        // into one that reloads the pointer from memory every character
+        if (current == ' ' || current == '\t' || current == '\n' || current == '\r')
         {
-            get_ignoring_pending_unget();
+            do
+            {
+                get_ignoring_pending_unget();
+            }
+            while (current == ' ' || current == '\t' || current == '\n' || current == '\r');
         }
     }