At first only some parts of the project are going to be based on blockchain. The rest will be transferred later. Isnt it going to make the platform slower?
No.
Most of the IM app are using the XMPP server for multi chat, video, voice message. XMPP is a well established and stable for IM.
The current blockchain technology still unable to support the IM features at the present day, we wrote an article for this, you may refer to our article for more details at
http://blog.kahnchat.com/kahnchat-app-bridging-scalable-gaps-blockchain-driven-crypto-world/The app is just a User Interface (UI), it does not process anything on the app/smartphone but on the server, the server responds the query to the app for display. The app is communicating with the different server, XMPP server, different blockchain algorithm installed server.
When the user using the social networking, the app does not need to have full connection with the blockchain server, a minimum API connection for the transaction push notification, thus the app is not heavy in term of it load.