G2H's Kingdom Runs has the channel #bfdia_5b, which was the main place to discuss speedruns in the past.
I have opened #5b-speedruns on the 5b editing Discord today. You can talk about speedruns, challenge runs, TASes, and other superplays there. https://discord.gg/qtePFSH
edit Aug. 27, 2020: 5b editing Discord has been much more popular since I've made this post. It's now the main hub for speedrun discussion, so I've changed the Discord link to it on the sidebar.
edit July 6, 2022: New invite to G2H's Kingdom Runs #bfdia_5b https://discord.gg/8sRSyWRMMu