Without drawing a background and disabling the blue backdrop graphic it would just be connected to the IRC channel and collecting messages similar to what the browser would be. As for SLOBS chat capture, SLOBS might do something fancy with emotes, especially if it loads FFZ or BTTV. For this program Twitch global emotes URL addresses are pre-loaded into a collection on start, so it does not store the graphics, just the text addresses.
To try to answer the question, this might contend with SLOBS chat capture for lightness if only using this program's chat function, not a large background or custom username images. For browser capture, I am not sure as some browsers do functions beyond the scope of the function the window serves, like extensions, sandboxing, or maybe various networking tasks. But for just capturing chat I do not see a browser being too heavy, although the comparison is sort of unknown.
