I'm answering my own question. Perhaps it will be useful for someone else.
Short answer: You can short-circuit the page generation process via wp_die() or wp_send_json().
But since I don't want to send a 500 status it eliminates wp_die(), and since I want to send html not json, it eliminates wp_send_json(). So instead, I copied & modified the wp_send_json() source to produce the following:
function wp_send_html( $response = null, $status_code = null ) {
@header( 'Content-Type: text/html; charset=' . get_option( 'blog_charset' ) );
if ( null !== $status_code ) {
status_header( $status_code );
}
if ($response) {
echo $response;
}
die;
};
In my code, after I determine the request is unchanged from the prior request, I call:
wp_send_html(null, 304);
... which sends a 304 response including any html response headers that have setup, and an empty body.
In the single post places that I'm using this, I've decreased response times from about 4-5 secs, down to 1.0-1.3 seconds.
Longer answer:
From @Mark's post, the primary things I need to consider are the post last change, and the last change for any theme or other global element. I am only handling only single page/post pages, and I've added a $MIN_UPD_DATE global at the top of my functions.php with I'll update when I do global/style changes (yeah, I know that's a little kludgey).
My more complete code in my functions.php is as follows:
// Update this on sitewide changes
$MIN_UPD_DATE = DateTime::createFromFormat('M d Y H:i:s', 'Sep 01 2018 01:01:01');
// For web pages and single post pages - note the last changed date
// Thanks to: https://wordpress.stackexchange.com/questions/172966/if-modified-since-http-header
// This will support clients sending HTTP header: If-Modified-Since
function handle_modified_since_header() {
global $MIN_UPD_DATE;
//Check if we are in a single post of any type (archive pages have no modified date)
if( is_singular() ) { // excludes multi-post pages
$post_id = get_queried_object_id();
if( $post_id ) {
header("Cache-Control: public");
// inherited/default was: Cache-Control: no-cache, must-revalidate, max-age=0
$postModTime = new DateTime(get_the_modified_time('D, d M Y H:i:s', $post_id));
if ($MIN_UPD_DATE > $postModTime) {
$postModTime = $MIN_UPD_DATE;
};
header("Last-Modified: " . $postModTime->format("D, d M Y H:i:s") . " GMT" );
return true; // can use modified date to expire page
}
}
return false; // can NOT use modified date to expire page
};
// Checks single post & page entries for a Post Modified Date
// after the http request header: IF_MODIFIED_SINCE
// or after the MIN_UPD_DATE
// else, on any missing elements, assumes that request is expired.
function is_request_expired() {
global $MIN_UPD_DATE;
$MOD_SINCE = 'HTTP_IF_MODIFIED_SINCE';
$DATE_FMT = 'D, d M Y H:i:s O';
$postModTime = null;
$isExpired = true;
$httpLastUpdate = null;
if (!isset( $_SERVER[$MOD_SINCE] )) { // Quit on no http last mod date
return $isExpired;
};
$httpLastUpdate = DateTime::createFromFormat($DATE_FMT, $_SERVER[$MOD_SINCE]);
if (!$httpLastUpdate) { // Quit on can't decode last mod date
return $isExpired;
}
$post_id = get_queried_object_id(); // Get Post last modified date
if( $post_id ) {
$postModTime = new DateTime(get_the_modified_time('D, d M Y H:i:s', $post_id));
};
if ($postModTime) {
// http last-mod-date is before post-list-mod-date or before min-upd-date
$isExpired = ($httpLastUpdate < $postModTime) || ($httpLastUpdate < $MIN_UPD_DATE);
};
return $isExpired;
}
function send_on_not_expired_single( $wp_query ) {
if ($_SERVER['REQUEST_METHOD']!=='GET') { // ignore for non-GET requests
return $wp_query;
}
$use_modified_since_rule = handle_modified_since_header(); // conditionally set Last-Modified http header
if ($use_modified_since_rule) {
$is_expired = is_request_expired();
if (!$is_expired) {
wp_send_html(null, 304);
};
};
return $wp_query;
}
add_filter( 'parse_query', 'send_on_not_expired_single', 200);
/**
* Send an HTML response back to an html request.
* patterned on the v4.9.8 WP codex wp_save_json
* from: https://core.trac.wordpress.org/browser/tags/4.9.8/src/wp-includes/functions.php#L3179
*
* @param mixed $response Variable (usually an array or object),
* then print and die.
* @param int $status_code The HTTP status code to output.
*/
function wp_send_html( $response = null, $status_code = null ) {
@header( 'Content-Type: text/html; charset=' . get_option( 'blog_charset' ) );
if ( null !== $status_code ) {
status_header( $status_code );
}
if ($response) {
echo wp_json_encode( $response );
}
die;
};
As a final note, per some comments/suggestions, I did review some caching plug-in's. Although some of them do have very good 'last-modifed-date' checking/updating logic (which I don't), the ones that I reviewed either specifically exclude logged in users from caching, or have significant limitations for logged in users which eliminate their usefulness for my focus.