What should a beginner focus on to improve FAST?


Learn exactly how to think in the opening, middlegame and endgame — this is what I teach.
Always blunder-check your moves.
Solve tactics in the right way.
Analyze your games.
Study games of strong players.
Learn how to be more psychologically resilient.
Work on your time management skills.
Get a coach if you can.


Tactics first and foremost, and analysing all your games second.
The proper way to analyse a game is another subject of discussion.

Learning to identify and prevent hanging pieces (free material for your opponent) is crucial. Developing your own 'scan' of the board takes time similar to how a pilot has their own way of scanning an instrument panel and making fast calculations with the data. For a chess player the thought process needs to be refined which is the basis of candidate moves:
https://en.wikipedia.org/wiki/Candidate_move
Write down a checklist to help you remember key points and eventually you wont need it anymore. I made one to remind myself to look out for enemy pawn movements which could be a double attack on my pieces and make sure all my pieces are protected (not hanging) from attack or being overloaded. Nothing worse than throwing a game away because I left a piece hanging.
https://www.chess.com/terms/hanging-piece-chess
With every move you make think if you have any checks, captures or threats but don't automatically check the enemy king if there is no value or if the enemy can block the check and develop their pieces at the same time.
Not giving away free material and reading up on tactics is a good way to start getting to middlegames against much better players and from there you can find your own style through practice and get to endgames.
https://www.chess.com/article/view/chess-tactics

Improving Your Chess - Resources for Beginners and Beyond…
https://www.chess.com/blog/RussBell/improving-your-chess-resources-for-beginners-and-beyond
The most important fundamentals for the beginner-novice to study in chess are: Tactics, Endgame and Opening principles. While the last of these is certainly important to understand and to become skilled at, you can never study too much of the first two of these. That is, time spent studying and improving your tactical and endgame skills will accrue more to your chess success and rating than focusing primarily on learning specific openings.
Unfortunately, endgame proficiency is probably the most neglected part of the game (at all levels of chess). By developing proficiency in the endgame you will also develop an understanding of when heading for an endgame (called 'liquidation') could improve your chances of winning (or drawing) a game that you might otherwise lose. The most important aspects of the endgame to become skilled in are pawn endings (along with the techniques of 'Opposition' and 'Triangulation') and rook endings,