Rendered diffs contain a lot of elements, and at times (during setup,

navigation, and resize handling, we have to query for specific elements.

Some of the queries we had were very inefficient, impacting these

operations. Most users wouldn't have noticed this, but there is a

measurable impact that can cause real issues with very large diffs.
One of the problems had to do with the use of the :first filter for

jQuery selectors. This filter is equivalent to grabbing the first result

from a query, but does not stop the query at the first result. We can

querySelector() instead to get the result we want. This makes a huge

difference.
Similarly, our parent element lookups now use closest(), instead of

parents(), giving us another performance boost.
Queries for anchors with a given name now use

document.getElementsByName(), which is much faster. This improves anchor

navigation.

Tested all forms of navigation:

Clicking chunks
Keyboard bindings
Chunk and file anchors in the diff index
Hashes in the URL (for files, chunks, and line numbers)

Tested expanding/collapsing chunks.
Measured performance for all the selectors using large diffs, and saw

noticeable decreases in time.