diff --git a/src/TinyGsmClientBG96.h b/src/TinyGsmClientBG96.h
index 11096785..a123f868 100644
--- a/src/TinyGsmClientBG96.h
+++ b/src/TinyGsmClientBG96.h
@@ -484,7 +484,7 @@ class TinyGsmBG96 : public TinyGsmModem<TinyGsmBG96>,
    * NTP server functions
    */
 
-  byte NTPServerSyncImpl(String server = "pool.ntp.org", byte = -5) {
+  byte NTPServerSyncImpl(const String& server = "pool.ntp.org", byte = -5) {
     // Request network synchronization
     // AT+QNTP=<contextID>,<server>[,<port>][,<autosettime>]
     sendAT(GF("+QNTP=1,\""), server, '"');
diff --git a/src/TinyGsmHttpsA76xx.h b/src/TinyGsmHttpsA76xx.h
index a10ea3b9..01cf8169 100644
--- a/src/TinyGsmHttpsA76xx.h
+++ b/src/TinyGsmHttpsA76xx.h
@@ -269,7 +269,7 @@ class TinyGsmHttpsA76xx
         return -1;
     }
 
-    int https_post(String payload)
+    int https_post(const String& payload)
     {
         return https_post((uint8_t *) payload.c_str(), payload.length());
     }
diff --git a/src/TinyGsmNTP.tpp b/src/TinyGsmNTP.tpp
index aaeda7e7..5af2ac91 100644
--- a/src/TinyGsmNTP.tpp
+++ b/src/TinyGsmNTP.tpp
@@ -21,7 +21,7 @@ class TinyGsmNTP {
    */
 
  public:
-  bool TinyGsmIsValidNumber(String str) {
+  bool TinyGsmIsValidNumber(const String& str) {
     if (!(str.charAt(0) == '+' || str.charAt(0) == '-' ||
           isDigit(str.charAt(0))))
       return false;
@@ -32,7 +32,7 @@ class TinyGsmNTP {
     return true;
   }
 
-  byte NTPServerSync(String server = "pool.ntp.org", byte TimeZone = 3) {
+  byte NTPServerSync(const String& server = "pool.ntp.org", byte TimeZone = 3) {
     return thisModem().NTPServerSyncImpl(server, TimeZone);
   }
   String ShowNTPError(byte error) {
@@ -54,7 +54,7 @@ class TinyGsmNTP {
    * NTP server functions
    */
  protected:
-  byte NTPServerSyncImpl(String server = "pool.ntp.org", byte TimeZone = 3) {
+  byte NTPServerSyncImpl(const String& server = "pool.ntp.org", byte TimeZone = 3) {
     // Set GPRS bearer profile to associate with NTP sync
     // this may fail, it's not supported by all modules
     thisModem().sendAT(GF("+CNTPCID=1"));