This is the reason I don’t run cabal update so often. You have to wait for both cabal and hls to fully build your project after it… It makes developing not fun… which is the main reason I programm in Haskell. Of course, cabal should not aim for fun but for stability and reproducibility, so I guess the correct behaviour is the current.
Nevertheless, I could imagine cabal build --develop to be a “just take the plan for whatever is downloaded or cached.” But now, we are approaching again to this other thread