I have been playing D&D 5e for only five months. I play a 5th level wizard. My warlock party member constantly berates me for using spell slots in encounters and says I need to "understand action economy."
I have read many articles on action economy and none say anything about saving spell slots and relying on cantrips only. I also have never run out of slots completely and only one party member has ever had to make death saving throws (me; I was in a bad mood and wanted my character to die so I could quit playing, so I "forgot" to move back behind cover).
Tonight I used Detect Thoughts and my familiar and knew there were only two orc casters to fight. We had them in a choke point, so when one appeared I threw a fireball at it on both my turns and killed it. On subsequent turns when the second enemy appeared, I threw two second level spells at it and it was easily defeated. We took only four total damage for the entire party.
I do not understand why I am being told I do not understand combat and "action economy" and that I should only ever use spell slots to "turn the tide." Why walk into the tide if you don't have to?
Best Answer
What they mean
Your friend is concerned that once you need to fight more than one or two encounters per (in-game) day, you will be frustrated not to have spells available to you. Action Economy is absolutely not the correct term for this; Action Economy is how much you can get done in a round (typically Action, movement, potential Bonus Action). What they mean is closer to 'trigger discipline' and is often called resource management.
In their defense, it is a fair thing to express to new players who may not realize the full potential of their slotted spells or the consequences of running out. However, while it is fair to notify a fellow player that they may wish to hold something back for an emergency, it is not OK to berate them and try to "play their character" for them.
This is exacerbated by your playstyle fitting with your GM's plans well; when you fight only once or twice between getting your spells back, there is very little reason to hold anything in reserve. In your friend's other groups they are probably expected to fight 4, 5, 6 or more combats per game day and are projecting that onto this group.
What you should do
Generally, there are a few ways to address this problem depending on how bad it is and your comfort level communicating to the other players.
It sounds, to me, that your friend thinks they are doing you a favor and the first option is the most appropriate. I personally prefer to have such talks just before or after a session and I find that most players will at least acknowledge and make a best effort to back off when it is expressed that they can be done "helping".
Bonus
You should never have to get your character killed to leave a group or even miss a session. Just tell your GM you need a break, and they should have no problem having your character disappear into the background temporarily while you get your head on straight. Sometimes it means the group takes the night off or does something else, but the game should not be running with unwilling players.
You don't have any responsibility to hurt yourself for their entertainment, only to minimize intentionally interfering with their fun.