There are various performance improvements being worked on for Drupal core, but it can be hard to keep track of how they do or don't fit together.
JavaScript and CSS aggregation/minification
Aggregation outside the main page request: #1014086: Stampedes and cold cache performance issues with css/js aggregation
JavaScript minification with Peast: #3302755: On-the-fly JavaScript minification
Query string compression #3303067: Compress aggregate URL query strings
Replace weights with dependencies: #1945262: Replace custom weights with dependencies in library declarations; introduce "before" and "after" for conditional ordering
defer/async aggregate support: #1587536: JavaScript aggregation should account for "async" and "defer" attributes
Image loading
Width and height for responsive images: #3192234: Apply width and height attributes to allow responsive image tag use loading="lazy"
Responsive image lazy loading: #3167034: Leverage the 'loading' html attribute to enable lazy-load by default for images in Drupal core
UI for loading attribute: #3173180: Add UI for 'loading' html attribute to images
fetchpriority=high: #3322289: [meta] Add fetchpriority=high on certain images/oembeds
Media thumbnail formatter: #2947796: Responsive image format for media
Media entity reference formatter: #3333448: Direct formatter for media items
Theme logo handling: #3345259: Theme logo image is rendered without dimensions
Image compression
Responsive image fallback formats: #3213491: Add fallback format support to responsive images
AVIF support: #3202016: Let GDToolkit support AVIF image format
Use webp in standard profile image styles #3275557: Add webp image conversion to core's install profile's image style
AJAX
Improve AjaxPageState: #3279206: Dynamically determine ajaxPageState based on libraries
Allow GET AJAX requests: #956186: Allow AJAX to use GET requests
Render caching
Abstract render cache and improve hit rate: #2551419: Abstract RenderCache into a separate service that is capable of cache redirects in a non-render array-specific way
Use bundle cache tag in views #3055371: Use new cache tag ENTITY_TYPE_list:BUNDLE in Views to improve cache hit rate
