Many experts and masters and GMs have mentioned how critical it is for beginners to focus on tactics, because it is usually is the weakest link in their performance improvement. Yet, many players have not listened to this carefully. Or they have become confused because of the complexity in the available aspects of each tactics skill and positional skill itself.
We have heard masters telling beginners to focus on tactics or something that makes them hang pieces. BUT, have we heard GMs telling masters what they should do to improve??? Can you masters explain your weakest link??
At the risk of becoming someone who cannot walk the talk, I will express my opinion. The basic premise of this (in)sanity is because I believe that in chess there is KNOWLEDGE and there is SKILL. You can become a strong player by skill alone, but it will not bring you too far. Knowledge without skill is worse, but is not something that is illogical (you can have good knowledge without good skill).
I have found some masters have difficulty to improve, stuck in 2000-2200, and think they cannot improve anymore. This is totally wrong. Usually the potential is still way above their current strength.
So I will explain my observation about one critical weakest link of masters and experts (and the reason why)...
It is in the strategy, in how to build a plan, in understanding a position from a higher level. What lies behind capablanca's words is deep (remember, he was very unlikely addressing his words to beginners, he was probably addressing his words to even GMs of that time):
“In order to improve your game, you must study the endgame before
everything else, for whereas the endings can be studied and
mastered by themselves, the middle game and the opening
must be studied in relation to the endgame”
I have mentioned somewhere that players who has employed the Caro-Kann somewhere during his development stage (around expert level), will most probably have no difficulty to cross the master level. Why? Because playing the Caro-Kann requires the player to play the opening and middlegame in relation to the endgame.
It is not the endgame skill that is important here, but the thinking process to build a plan based on endgame knowledge (and general understanding of any positions of course). Once this skill is achieved, it can actually be implemented from any openings you choose, not necessarily a Caro-Kann, but playing the Caro-Kann will help you build this skill.
Now I will tell you the reason why masters have problem here... Because you cannot find anywhere in books or internet how GMs really really think when he evaluates a position and builds his plans.
Pogonina didn't think that there is a secret (but she at least agreed with me that there are secrets in opening preparation), but I will tell you that many GMs will not tell you what will make you to become their competitor. What GM will say is as shallow as what has been said before (My System, How To Reassess Your Chess, etc).
I have gone through all annotated games of highly rated players in gameknot.com, and I have read almost all annotated games in chess.com articles (I will read them all). What have I found? The strongest player that is willing to share important thinking process is IM David Pruess. He deserves to become a GM someday, but where are the others who knows better? They keep silent because they know its a rare skill.
To make this understandable to beginners who read this, I will explain a basic skill how an endgame knowledge can affect your middle game plan. I always pay attention to pawn split. I need to know where my pawn majority is, on the Kingside or Queenside, and I will play based on my pawn endgame knowledge, as one of my winning asset. If it doesn't go to the endgame stage, fine. But if it does, I have made sure that I will win it.
If you know how a Rook will work better with a Bishop, or how a Queen will work better with a Knight (sometimes better with a Bishop!) in an endgame, it will affect how you decide in your middlegame plan.
Let me show you a more complex position, which is a Sicilian Dragon position. Whether my analysis is right or wrong, I believe you have never heard this analysis before:
I believe that the Dragon is refuted through this 9.0-0-0 Yugoslav Attack. But many will disagree.
It is a common knowledge that White has a "push the h-pawn, sac, sac and mate". This is a threat that will at least balances any other threat from Black. But that is not what makes the Dragon refuted. If you look at the position carefully, the position is almost equal. The only thing that Black doesn't have is the winning Queenside pawn endgame asset that White has. White should maintain and pay attention to this asset, because when both sides play correctly (avoid tactical mistakes), the only refutation to Black position is the Queenside pawn endgame.