url_to_postid method in walker_nav navigation menu causing exceptionally high TTFB

I can’t say for certain, but I’d wager that url_to_postid() is simply not a very fast function. You shouldn’t need to use it though, since the original post ID is available from $item:

if ( 'post' === $item->object ) {
    $post_id   = $item->object_id;
    $thumbnail = get_the_post_thumbnail_url( $post_id );
}