Spotlight time is any time in which you are the focus of the scene in my opinion which necessarily includes however long it takes to resolve your turn.
Ah, a difference in definition. Spotlight time is times when you get to act cool and amazing, when you stand out - have the spotlight on you. If one character is the focus of the session with everything revolving around them, they had the spotlight. The fact that wall time was spent on other characters isn't really an issue.
Since we're talking about time in combat, let me give a combat focused example. Say a 11th level character with a party of 2nd levels went on a generic dungeon crawl. The 10th level pulled out spells, tanked bosses, and generally made themselves the star of the show. They will have taken the majority (if not all) of the spotlight, regardless if their turns took longer or shorter than any others.
But really, I don't think we need to agree on the definition of spotlight to move on.
Players have a right to their particular concept within the bounds of agreed upon rules and genre in my view, but that right comes with a corresponding responsibility: Share the spotlight more or less equally with your fellow players over the course of the session. If someone isn't doing that, I would say it's perfectly reasonable for a fellow player or DM to say something about it and work toward a solution.
For example, my circle of the shepherd druid, Bo-Peep, has a conjure animals spell in her retinue which she uses to summon a flock of sheep (goats, reskinned). Rather than bust that out on regular, I do it only when it would be particularly useful or very cool in the scene. Because I know that it will take longer to resolve my turn than normal, I'm judicious in its use as I see it as my responsibility to share spotlight more or less equally. That's just being a conscientious player in my view, respectful of other people at the table.
If I ignore the word "spotlight" and replace it with "time spent" I can see where you are coming from. You're talking courtesy to keep turn length down. I'm all for that. I would do everything I could to manage that - have things statted up, have enough d20s to roll them all at once, use average damage if allowed, etc.
On the other hand, just like a single attack will take less time then an area of effect, some things are in the realms of the overhead the rules put on us.
If the rules had, for example, an easy minion system that made running a bunch of lower CR summons very simple and quick, that would be a great thing. But 5e treats them as full combatants, which takes as much time to resolve both their turn, and also actions against them, as it would for any other being in a combat.
I don't think it's invalid to play a standard archetype, one supported by the system and even having some subclasses the specialize in it like the Sheppard druid. I expect the player to do everything they can to keep it moving as fast as they can, just as I'd expect any other player. (Casters: look up your dang spells BEFORE it is your action). I can't hold that the mechanics of the system being cumbersome against them though.