I was super skeptical, but I do see enough going on to suggest that Epic is doing something actively that takes more CPU than Steam. I mean, ignoring temps for a sec the difference in Task Manager is pretty direct: ~3% (Epic) vs. ~0.1% (Steam).
But hey, I have a 5800X so let's do a super quick/not rigorous test! CPU Tctl/Tdie Current Temp in HWiNFO64 after idling for two minutes:
- Windows Baseline (Epic/Steam not running): ~50 C
- Steam Running (Minimized to Tray): ~54 C
- Epic Running (Minimized to Tray): ~64 C
- Steam Library open (Epic is closed): ~58 C
- Epic Window open, but hidden behind Chrome: ~66.5 C
I did notice 72.5 C if you idle on the Library page, but that's as the active window - losing focus (like, say, putting a Chrome window overtop) drops the temperature immediately.
tl;dr yep, Epic uses more CPU even in the tray, which explains why temps are higher. Temps != performance though, so I can't say if this does anything other than simply make your CPU a bit hotter. Could be just the 5800X likes to aggressively ramp power stages?