warn about performance when eventlet/gevent are not installed
warn about performance when eventlet/gevent are not installed

njamaleddine/Flask-SocketIO,miguelgrinberg/Flask-SocketIO,jwg4/Flask-SocketIO,Grant1219/Flask-SocketIO,Grant1219/Flask-SocketIO,jwg4/Flask-SocketIO,miguelgrinberg/Flask-SocketIO,njamaleddine/Flask-SocketIO