looks like main tasks are performed via core3 and core 4 and those are not utilized fully so they could take over the tasks from the other performance cores too. Efficiency cores are doing the OS background stuff.That's not true at all, most apps support multi-threading, and the OS supports multitasking at the lowest levels to distribute work across CPU cores.
Here's my Activity Monitor CPU history for my M1 Pro browsing a handful of websites with just Safari open browsing different websites on just two tabs. All 10 cores are being used in some capacity (albeit diminishing amounts for cores 5-10). Again that's just Safari with two tabs open browsing regular mainstream websites.
View attachment 2370470
This supports my theory.