By ‘bad’ I’m guessing more than an hour less battery life? And by ‘Intensive’ I’m guessing playing games right?
The difference in performance was pretty low few years back, I think S810 era had better Exynos SoC as S810 was overheating & throttling a lot. Sadly throttling is a major issue this year. For few benchmark runs, S20 exynos holds up close to S865 but with time, performance falls below S855 level according to some tests.
Say out of 6 hrs SOT, if a phone lasts 1 hr less, that's a significant downgrade. Stand by times will be affected almost similarly.
This year, the situation is surely worse than it was earlier. That's why many videos & articles regarding poor Exynos SoC vs SD, esp by Europeans, who do spend on flagship phones. I think OP8 Pro can actually steal customers away from Samsung in the EU. Unless Exynos can bounce back next year, it will surely hurt Samsung a lot.
I do have a theory that Exynos SoC phones degrade faster in long run, in terms of performance than SD ones. Not much solid evidence. My father's Samsung C7 with S625 is still decently smooth whereas an uncle of mine has A7 (2017). I see more stuttering in A7 than C7 after close to 3 years. I know both phones had different usage history, but that's as close of a comparison I can get. Sadly don't know anyone with S8 exynos.