I'd say a bit of both. People who are fine with having a DOM worth of overhead don't give me much confidence they are selecting algorithms well, which can absolutely affect performance and efficiency regardless of what language they are written in.

Hell, even with the optimal data structures and algorithms selected, how you set them up can make a huge difference.

Like I once had essentially a data translator that, in order to be generic, used keys in a dictionary to replace the hardcoded data structure that needed to be updated any time the spec changed, even if we didn't care about the new values.

The first version used an array of dictionaries and was slow as fuck. Each line of data would get its own dictionary and just parsing the data into this scheme took a couple minutes IIRC. Since the data was guaranteed to have the same keys each line (we were worried about changes between versions, not lines), I changed it to a dictionary of arrays, where each line would have the same index across the dicts, and it brought parse time down to several seconds. Huge difference even though big O notation would have simplified to the same expression for the two methods.

But if you pick something with a worse big O expression, it gets worse on a whole other level.