That's normal. Everytime Yappari connects, i.e. also when you start it, it syncs your contacts which makes Yappari seem to hang. For me I am having around 400 contacts with 1-3 phone numbers each and it takes 1 to 2 minutes to become responsive after starting it. Just be patient and wait at least 5 minutes, DO NOT KILL IT! Depending of the number of contacts you are having, this might take a lot longer, too. If you were following the logs at the same time you would see, that it's sending chunks of contacts to the server to check if they are using Whatsapp or not.