First of all, please make sure that your remote and host machines meet our system and bandwidth requirements:
http://www.coderebel.com/support/faq/what-are-the-system-requirements-for-irapp

Otherwise, speed is dependent upon your network configuration.  To increase rendering speed, we recommend that you reduce the resolution, color quality, or try our iRAPP Client in RDP/iRAPP modes. 

You can reduce image quality by going to Options > Experience Tab.  Sometimes it also helps to set your background image to a solid color.