Image scaling is computationally expensive. Most video display operations on PCs pass that work to the display adapter’s GPU through native methods. It seems the IP camera component can’t access those methods, meaning it will never be fast. Java provides a great multi-platform environment, but it is hard to accommodate native interfaces.
Consider running a streaming video server technology alongside Ignition that can natively scale to your desired resolution outside of Ignition. The streaming server would subscribe to the raw feed, possibly in an efficient rtsp format, then supply it to local consumers in the desired scaled MJPEG format.