Java Memory Startup Options
Twitch launcher forcibly adds the -Xms256m java argument, disallowing the user to set their own, and generally reducing MineCraft's performance. -Xmx sets the maximum java can use and reserves it from system memory anyway. -Xms is the starting amount java loads with, the difference being left "reserved but unused".
Suggestion: Twitch Launcher's jvm arugments -Xms should copy its value from the -Xmx slider, have its own slider, or just be skipped completely to allow users to set it via the custom arguments.