This is super surprising to me, because irccloud is exactly what we use at Mozilla. Everyone seems happy with it's performance, and obviously most people are using FF.
If it helps, I'm running irccloud's "Ash" theme, with the sidebar on the left, monospace font, "nicknames on a separate line", "user icons" and colored nicknames all enabled.
It's pretty bad on both Windows 10 and Arch Linux (different computers). Worse on Windows, actually, but only after some time -- I'm in ~30 channels, fwiw.
As always it's probably a plugin. You could integrate the functionality of plugins which everybody uses, such as an adblocker (Instead of pocket? You are already doing it with the developer tools) -- but you get all your money from Google...
Tracking Protection is pretty much an ad-blocker. It's default-enabled in Private Browsing and you can enable in normal browsing, too, by setting "privacy.trackingprotection.enabled" in about:config to true.
