diff --git a/src/libslic3r/Config.cpp b/src/libslic3r/Config.cpp
index 42d6970ad1..427f6e66c9 100644
--- a/src/libslic3r/Config.cpp
+++ b/src/libslic3r/Config.cpp
@@ -849,6 +849,19 @@ ConfigSubstitutions ConfigBase::load_from_json(const std::string &file, ForwardC
     return std::move(substitutions_ctxt.substitutions);
 }
 
+// Case-insensitive compare of a JSON key against a fixed ASCII one, without
+// boost::iequals, whose std::locale() takes a lock the whole process shares in the
+// MSVC runtime.
+static bool ascii_iequals(const std::string &key, const char *literal)
+{
+    auto lower = [](char c) { return (c >= 'A' && c <= 'Z') ? char(c - 'A' + 'a') : c; };
+    size_t i = 0;
+    for (; i < key.size() && literal[i] != '\0'; ++ i)
+        if (lower(key[i]) != lower(literal[i]))
+            return false;
+    return i == key.size() && literal[i] == '\0';
+}
+
 int ConfigBase::load_from_json(const std::string &file, ConfigSubstitutionContext& substitution_context, bool load_inherits_to_config, std::map<std::string, std::string>& key_values, std::string& reason)
 {
     json j;
@@ -916,44 +929,44 @@ int ConfigBase::load_from_json(const std::string &file, ConfigSubstitutionContex
         }
         //parse the json elements
         for (auto it = j.begin(); it != j.end(); it++) {
-            if (boost::iequals(it.key(),BBL_JSON_KEY_VERSION)) {
+            if (ascii_iequals(it.key(), BBL_JSON_KEY_VERSION)) {
                 key_values.emplace(BBL_JSON_KEY_VERSION, it.value());
             }
-            else if (boost::iequals(it.key(), BBL_JSON_KEY_IS_CUSTOM)) {
+            else if (ascii_iequals(it.key(), BBL_JSON_KEY_IS_CUSTOM)) {
                 //skip it
             }
-            else if (boost::iequals(it.key(), BBL_JSON_KEY_NAME)) {
+            else if (ascii_iequals(it.key(), BBL_JSON_KEY_NAME)) {
                 key_values.emplace(BBL_JSON_KEY_NAME, it.value());
                 if (it.value() == "project_settings")
                     is_project_settings = true;
             }
-            else if (boost::iequals(it.key(), BBL_JSON_KEY_URL)) {
+            else if (ascii_iequals(it.key(), BBL_JSON_KEY_URL)) {
                 key_values.emplace(BBL_JSON_KEY_URL, it.value());
             }
-            else if (boost::iequals(it.key(), BBL_JSON_KEY_TYPE)) {
+            else if (ascii_iequals(it.key(), BBL_JSON_KEY_TYPE)) {
                 key_values.emplace(BBL_JSON_KEY_TYPE, it.value());
             }
-            else if (boost::iequals(it.key(), BBL_JSON_KEY_SETTING_ID)) {
+            else if (ascii_iequals(it.key(), BBL_JSON_KEY_SETTING_ID)) {
                 key_values.emplace(BBL_JSON_KEY_SETTING_ID, it.value());
             }
-            else if (boost::iequals(it.key(), BBL_JSON_KEY_FILAMENT_ID)) {
+            else if (ascii_iequals(it.key(), BBL_JSON_KEY_FILAMENT_ID)) {
                 key_values.emplace(BBL_JSON_KEY_FILAMENT_ID, it.value());
             }
-            else if (boost::iequals(it.key(), BBL_JSON_KEY_FROM)) {
+            else if (ascii_iequals(it.key(), BBL_JSON_KEY_FROM)) {
                 key_values.emplace(BBL_JSON_KEY_FROM, it.value());
             }
-            else if (boost::iequals(it.key(), BBL_JSON_KEY_DESCRIPTION)) {
+            else if (ascii_iequals(it.key(), BBL_JSON_KEY_DESCRIPTION)) {
                 key_values.emplace(BBL_JSON_KEY_DESCRIPTION, it.value());
             }
-            else if (boost::iequals(it.key(), BBL_JSON_KEY_INSTANTIATION)) {
+            else if (ascii_iequals(it.key(), BBL_JSON_KEY_INSTANTIATION)) {
                 key_values.emplace(BBL_JSON_KEY_INSTANTIATION, it.value());
             }
-            else if (!load_inherits_to_config && boost::iequals(it.key(), BBL_JSON_KEY_INHERITS)) {
+            else if (!load_inherits_to_config && ascii_iequals(it.key(), BBL_JSON_KEY_INHERITS)) {
                 key_values.emplace(BBL_JSON_KEY_INHERITS, it.value());
             }
-            else if (!load_inherits_to_config && boost::iequals(it.key(), BBL_JSON_KEY_INCLUDES)) {
+            else if (!load_inherits_to_config && ascii_iequals(it.key(), BBL_JSON_KEY_INCLUDES)) {
                 key_values.emplace(BBL_JSON_KEY_INCLUDES, it.value().dump());
-            } else if (boost::iequals(it.key(), ORCA_JSON_KEY_RENAMED_FROM)) {
+            } else if (ascii_iequals(it.key(), ORCA_JSON_KEY_RENAMED_FROM)) {
                 key_values.emplace(ORCA_JSON_KEY_RENAMED_FROM, it.value());
             } else {
                 t_config_option_key opt_key = it.key();