I've played the old edition but in any case people can see how things differ reading it, if you can't it is your problem.
So despite dodging the question, the answer is no, you haven’t actually tried either Greyhawk Initiative or Speed Factor Initiative in 5th Edition, as presented in UA or the DMG. That’s fine, if the default Initiative system works for you, more power to you in using it. There is, however, a continuing pattern of people who say the Greyhawk/Speed Factor system(s) are terrible and slow not actually having tried them, and people who have tried them saying that in their actual play experience, these systems are not significantly slower and in fact flow better than the default system.
To be a slave of decisions made previously is bad for roleplaying, it only makes things more boardgamey. People could bring their decisions for combats written from home if they think otherwise.
There is a world of difference between declaring an action at the top of the round, after seeing the current layout of the battlefield and hearing what the monsters are gearing up to do, as compared to making your decisions at home, away from the table, devoid of context. This is false equivalence and a fallacious argument.
Simulation is bad, good old rolemaster was better 30 years ago, taking into account things like position, weapon length, wounds, etc and actions had their own phases. Other systems have improved beyond that, on the other hand this one only brings random iniative variability per round and future chores for players.
Greyhawk Initiative and Speed Factor Initiative don’t take any of those things into account, and their primary benefit is the narrative pacing and flow of combat, not simulation. Interestingly, another pattern in this conversation is people who have not tried these alternate Initiative systems but claiming they’re terrible assuming that simulation is the reason people use them, while people who have actually tried them say that no, simulation is not the reason they like these systems, pacing is.
This is a RPG, I'm not sure who loves to narrate what he thinks and later how he can not do what he would like to do or how he can not react to current events because of what he thought previously.
This sentence is a little difficult to parse, but players do have the opportunity to react to the current situation on their turn under Greyhawk and Speed Factor. They are only locked into the decision of what action to take (and by default Greyhawk, if they want the option to move). Everything about where to move and how to use the action is still decided on the player’s Initiative count.
More decision stages slow things, unless the decisions get simplified and this is not the case, they are not changed to rock, paper or scissors. There is a good reason why the DM's book says this kind of initiative rule is slower but you are free to call the authors liars.
The decisions are in fact simplified. In standard Initiative you have to decide if and where to move, what action to take, and any necessary targets all on your turn. In Speed Factor Initiative you only need to decide which action to use at the top of the round, and decide on your turn if and where to move and any necessary targets. In Greyhawk Initiative you only decide what action to use and whether to move at the top of the round, and where to move and necessary targets on your turn. In all three cases you are making the same number of decisions per round, but in Greyhawk and Speed Factor you make a subset of those choices at one point and the rest at another, which means there are fewer variables to account for at the time of making each of those decisions. You also have to make some of those decisions with incomplete information. This means you do not have the luxury of weighing every possible variable and trying to plan out the ideal options, instead you have to be satisfied with something good enough and move on. Since you’re also doing this during a shared decision making time instead of personal decision making time, there is more pressure to make your decision quickly. Finally, I am not calling the designers liars. They said these systems were
more complex, not
slower. And they are more complex. I find the added complexity to be a worthwhile trade in exchange for better gameplay flow for some groups, and not worth it for others. It depends on the experience of the players and their familiarity with the system in general and with their characters’ abilities in particular. You may not find it worthwhile for any group, and that’s perfectly fine. But please do not call
me a liar for saying that, in much experience, these systems
do improve the flow of the game, while not noticeably impacting the total time it takes.
Most DMs don't want more work, a good reason behind the simplifications of this edition.
That’s their decision to make. Personally, if more work on my part will make the game better for the players, I’ll do that extra work, as long as the amount of extra work is outweighed by how much it improves the game. I think most DMs would agree, although everyone has different thresholds for how much payoff they consider worth how much work. In this case, I find these alternate Initiative systems to be only a small amount of work for a significant payoff, but only if the players are fairly experienced. Your mileage may vary, and that’s perfectly fine.