diff --git a/toolchain/sem_ir/inst_fingerprinter.cpp b/toolchain/sem_ir/inst_fingerprinter.cpp
index ac23f06a1c76..ff6dba6df04e 100644
--- a/toolchain/sem_ir/inst_fingerprinter.cpp
+++ b/toolchain/sem_ir/inst_fingerprinter.cpp
@@ -150,12 +150,12 @@ class StringFingerprintStore {
     bool first = true;
     for (const auto& item : contents_) {
       if (!first) {
-        result += ",";
+        result.push_back(',');
       }
       first = false;
       result += item;
     }
-    result += "}";
+    result.push_back('}');
     return SaveString(std::move(result));
   }
 
diff --git a/toolchain/sem_ir/inst_namer.cpp b/toolchain/sem_ir/inst_namer.cpp
index d3979edac16e..4c9d79991aa5 100644
--- a/toolchain/sem_ir/inst_namer.cpp
+++ b/toolchain/sem_ir/inst_namer.cpp
@@ -409,7 +409,7 @@ auto InstNamer::Namespace::AllocateName(
   }
 
   // Append numbers until we find an available name.
-  name += ".";
+  name.push_back('.');
   auto name_size_without_counter = name.size();
   for (int counter = 1;; ++counter) {
     name.resize(name_size_without_counter);