I've been considering the following homebrew magic item to give to my campaign's BBEG. The flavour/intent is that the villain is manipulating the way time works across the multiverse, and I designed the item around that idea. Note that I don't intend for my players to use this, so I'm more concerned about how this would affect the balance of an encounter, rather than abuse that the players might do.
Amulet of Quick Casting
Wondrous item, legendary (requires attunement)
The amulet has 10 charges and regains 1d10 expended charges daily at dawn.
When you cast a spell of 1st level or higher with a casting time longer than 1 Action, you can expend a number of charges equal to the spell's level, assuming the amulet has the necessary charges. If you do, the casting time of the spell becomes 1 Action.
I've gone over the list of spells with a casting time longer than 1 Action, and, while a couple of spells stand out to me as being overpowered if cast in 1 Action (namely, Simulacrum, Tsunami, Imprisonment and the various Conjure spells), the vast majority seem to have, at best, some combat utility, with the rest not being worth wasting charges to cast with a shorter casting time.
Because of this, I'm not too worried about this affecting the balance of any encounters involving my villain, especially as my players will (by the time they actually fight it) be around levels 12-15, and will have access to their own 7th and 8th level spells. Furthermore, in a single encounter, the charges means that the more "dangerous" higher level spells are likely only able to be quickened at most once.
Finally, given the spell-replication ability of Wish, this seems to be a slightly better (can cast 9th level spells as 1 Action) version of a 9th level spell, rather than anything incredibly powerful.
Therefore, for the rarity, and given the players' levels, is this likely to completely shift a campaign-concluding fight, when the BBEG is something akin to a lich or other high CR spellcasters, or is it likely to have little impact? Ignore any ramifications the item might have if it falls into the hands of player characters, this is entirely for "DM use".
Related question, discusses the impact of casting 1 minute spells in 1 round, but is about a homebrew game rule, not a magic item.