“time-consuming” does not cleanly encapsulate difficulty, because lots of easy things are time-consuming too.
Perhaps “slow to reward” is a better way to gesture at the phenomenon you mean? Learning a language takes a high effort investment before you can have a conversation; getting in shape takes a high effort investment before you see unambiguous bodily changes beyond just soreness. Watching TV and scrolling social media are both time-consuming, but I don’t see people going around calling those activities difficult.
To extend this angle—I notice that we’re more likely to call things “difficult” when our expectations of whether we “should” be able to do it are mismatched from our observations of whether we are “able to” do it.
The “oh, that’s hard actually” observation shows up reliably for me when I underestimated the effort, pain, or luck required to attain a certain outcome.