Flask with multiple requests post

I have a flask API that processes requests from a web application. If I send a single request to the flask API, the processing time is around 600ms. But if I send several requests simultaneously (100 requests), the processing time goes up to 14s. My goal is to optimize this processing time during several simultaneous requests. PS: I use waitress as deploy server.

My code:

@app.route('/form', methods=['GET', 'POST'])
def process_form():
   '''
   Prend les valeurs json envoyé par l'appli, retourne reponse json
   '''
   if request.method == 'POST':
      data = request.get_json()
      data['value1'] = (data['value1']) + 1
      return data