I have a few thoughts here.
Uboachan was not much (if at all) faster before the Discord was made, and the two communities have almost no interest in each other. They are separate things. Very few discord users use the boards and vice versa. Almost all of the Discord users came from outside the Imageboard community.
Currently the Sushigirl Discord has one channel mapped to the IRC channel, and 4 users (plus the staff). The Uboachan Discord server is a massive community with dozens of channels, and I have no such plans for the Sushigirl server. If I make a large Discord server, there is large potential for a dumpster fire to happen, and then I need a ton of staff, and I really don't want to deal with that. Uboachan's Discord is enough to handle.
I think the biggest concern you've brought up here is privacy and I'd like to see if anyone else is worried about that. Using IRC will prevent Discord from collecting any info other than your text though.
One of my biggest incentives for making the Discord server was a total lack of decent IRC support on mobile. All of the apps suck, and I am not sitting in front of a computer all day like I used to. I needed something with good mobile support to keep in contact with my moderation staff, so the moderation channel was bridged a while back, and then I bridged the main channel yesterday.
Another incentive for me was that I have to use special software on IRC just to be able to see what happened when I was not signed in, and that ultimately makes it a bad platform for keeping in touch with my moderators.
Anyway if anyone else has objections I would like to hear them, and it's possible I could roll back this change, but mostly I did it so people can chat from their cell phones and I am currently only planning to have the one channel.