yes, as javascript is mono-thread, blocking the browser in a long execution blocks also the communication with the server and so can cause the coonection to be reinitialized in POLLING.

The client library passes from STREAMING to POLLING in two cases:
  • the STREAMING connection can't be initialized (ie after opening the connection no data is received before a certain timeout).
    see also setBufferedStreamingHandled
  • there is a heavy load on the machine and the client tries to degradate to POLLING in the hope to diminish such load
    see setSlowingEnabled
HTH