Very strange problem during beginning with MKR Wifi 1010

Development Tools IDE 1.x
14

I'm hoping that this so simple and elementary thing should be explainable ...
I'm still digging into the MKR WiFI 1010 domain.
I wrote earlier that the command "Serial.read" is not running on this board. I finally reached succesfully my target, using "Serial.parseInt", but this command get my app very slow (can say unacceptably slow).

Hereafter a very simple example :

char d;
String Msg;

void setup() {
  Serial.begin(115200);
}

void loop() {
  while ((d = Serial.read()) > '\n') Msg1 += (char) d;

   if (d == '\n')
  {Serial.println(Msg1); }
}

This sketch cannot be compiled , and sometimes freezes the MKR, needing a "deep" reset.

Here the warning :

Le croquis utilise 12620 octets (4%) de l'espace de stockage de programmes. Le maximum est de 262144 octets.
Les variables globales utilisent 2220 octets (6%) de mémoire dynamique, ce qui laisse 30548 octets pour les variables locales. Le maximum est de 32768 octets.
processing.app.debug.RunnerException
	at cc.arduino.packages.uploaders.SerialUploader.uploadUsingPreferences(SerialUploader.java:152)
	at cc.arduino.UploaderUtils.upload(UploaderUtils.java:77)
	at processing.app.SketchController.upload(SketchController.java:732)
	at processing.app.SketchController.exportApplet(SketchController.java:703)
	at processing.app.Editor$UploadHandler.run(Editor.java:2055)
	at java.lang.Thread.run(Thread.java:748)
Caused by: processing.app.SerialException: Erreur de la commande « touch » sur le port série « COM8 ».
	at processing.app.Serial.touchForCDCReset(Serial.java:107)
	at cc.arduino.packages.uploaders.SerialUploader.uploadUsingPreferences(SerialUploader.java:136)
	... 5 more
Caused by: jssc.SerialPortException: Port name - COM8; Method name - openPort(); Exception type - Port busy.
	at jssc.SerialPort.openPort(SerialPort.java:164)
	at processing.app.Serial.touchForCDCReset(Serial.java:101)
	... 6 more
processing.app.SerialException: Erreur d'ouverture du port série « COM8 ».
	at processing.app.Serial.<init>(Serial.java:152)
	at processing.app.Serial.<init>(Serial.java:82)
	at processing.app.SerialMonitor$2.<init>(SerialMonitor.java:132)
	at processing.app.SerialMonitor.open(SerialMonitor.java:132)
	at processing.app.AbstractMonitor.resume(AbstractMonitor.java:132)
	at processing.app.Editor.resumeOrCloseSerialMonitor(Editor.java:2120)
	at processing.app.Editor.access$1300(Editor.java:117)
	at processing.app.Editor$UploadHandler.run(Editor.java:2089)
	at java.lang.Thread.run(Thread.java:748)
Caused by: jssc.SerialPortException: Port name - COM8; Method name - openPort(); Exception type - Port busy.
	at jssc.SerialPort.openPort(SerialPort.java:164)
	at processing.app.Serial.<init>(Serial.java:141)
	... 8 more
Erreur d'ouverture du port série « COM8 ».
Erreur durant la définition des paramètres du port série: 115 200 N 8 1

How can we use an IOT board which is unable to communicate decently via UART ???