My DM insists on the combat being 10 seconds per turn and the round time to be the sum of all turn times (so a minute for 6 participants, for example). I am aware that this is not how turn/round time normally works.
This makes many spells with 1 minute durations like Hold Monster last at most a round.
I am playing a Fiend Warlock and I am worried it'll seriously underpower my character compared to non-spellcasters. Many spells available to me are supposed to last up to encounter-long (Hold Monster, Hunger of Hadar, Vampiric Touch, Hold Monster, Eyebite to name a few). Am I rightly worried, or overreacting? Are other spells sufficient for me to carry my weight?
I am aware of PHB's ruling on this and that rounds normally don't sum to turns. I brought it up in the conversation with my DM but it's his idea and he said that it has to be this way or the campaign's BBEG would be too strong. To me it sounds like a problem with balancing the BBEG and breaking other things is not the way but I wanted a second opinion on how this affects the game.
Best Answer
Yes and No. But mostly Yes.
It makes spellcasters worse, by making some spells worse. In particular, the spells you mentioned, i.e., spells that rely on a Duration. Notably, however, other spells are completely unaffected - Eldritch Blast, Fireball, or anything else with Duration: Instantaneous.
Furthermore, not only spellcasters get screwed. The Barbarian rage now lasts one round. Yay, not amazing. I would never, ever play a Barbarian in this table. If you ever get to 20th level playing like this, Paladin's 20th level features are now probably worse than any of their other features. Same goes for some Channel Divinity features. In short, it screws anything that has a duration given in time.
On the other hand, you now can cast Conjure Fey in just one round! Yay! That is to say, this goes the other way as well: spells with a Casting Time of one minute now can be cast in one round, and usually these spells are not balanced to be cast during the combat, so that may actually break the game in your favor. I'm not sure for Warlock, but it certainly can get some Wizards and Clerics happier. Animate Dead on the enemy you just killed? Neat.
Honestly? Yes. Especially as a Warlock. That doesn't mean the ruling makes any sense. It completely breaks the foundation on which the features and spells were designed, and probably your DM doesn't fully understand how much this change impacts the game. See this related question on how to Talk to your DM about their ruling being harsh.
You mentioned in a comment that
And I really think this is worth tackling on. Your DM is creating his own problem, then trying to solve it by creating an even larger problem, and destroying the game foundation by doing that. If the BBEG is too strong under the usual rules of the game, then he should be re-balancing the damn BBEG, not changing the entire system to fit that one NPC.