I guess my concern about subtrees is that they seem like they can result in unnecessary duplication / use of disk space. But the ability to work offline with a complete copy of the code seems worth the tradeoff in most cases, and git-lfs should help deal with large file sizes.
Have you run into any "slow push speeds" with subtrees, as the person complains about in the first article I linked?
No, I haven't run into any such issue, though I have only used it with small to medium sized repositories. The repo simply grows linearly with the size of the secondary repository. With a submodule, you would have to clone it just the same. In fact, you might enjoy some benefits from cross-compression by using subtrees that would not be available to submodules, and it's faster to reuse the same clone connection you were already getting the source from.
Have you run into any "slow push speeds" with subtrees, as the person complains about in the first article I linked?