1) Anything cosmetic gets a "nit" added to the comment.
2) When a review looks like mostly assets or boiler plate, I look through quickly for anything glaring and approve while noting my assumptions that the author has run the build and verified the additions.
3) If I'm in a hurry and the author is available I grab them and do an over the shoulder review. You might be surprised how much time this saves.
4) Don't struggle with difficult sections of other people's code unless you have a vested interest in remembering this specific section of code. Ask about unclear areas.
5) Trust your CI process. Don't try to consider every edge case for your reviewee.
6) Develop a culture of small work units. Encourage people to integrate every day or two. This means reviews are small and easier to grok.
Code reviews are not a guarantee of correctness. They are another tool. We should find a good trade off between effectiveness and cost to get the most value out of them not strive for perfection.