This is a bit old, but one of the first results on Google and neither the actual reason nor the workaround seem to be explained, so here they are: on first get(), the network manager will lazily load some library (DLL), which can take quite some time and freeze the UI if the code expects (rightly) that get() is purely asynchronous and will return quickly. The workaround is to pre-connect to the given server with connectToHost() or connectToHostEncrypted() earlier. After doing so the first get() has the same performance than subsequent ones.
Posts made by djee
RE: QNetworkAccessManager - first GET very slow