What is the speed of the internet connection, its not likely to be a switch or router issue but a speed issue on the WAN side of things. Also the more cameras you try to view at once the increase in bandwidth.
If you are running high FPS with high resolution you are going to run into problems viewing externally. Many cameras support a second lower quality stream for this purpose however you are still streaming usually poor encoded video which is nowhere near as efficient as say Youtube.
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