I will expand the other answers a little, because I think they are all situational, as I will explain below.
How important is that information?
And, more importantly: how crucial is it to not give to other players? (this is the equivalent question to "How likely is it that the player will instantly give this information to the rest of the party anyway?")
If the answer is "not much" for any of these two questions (or "a lot" for the equivalent second question) - state it for everyone to listen. In your examples, first and second bullets don't seem to need that much secrecy.
I've been doing it with a group with some new players and actually using it to remind them what their characters know. One PC finds a track, I describe it, another player makes question about it - I remind them that they don't know about this track yet.
A huge pro of it is that, when the player chooses to give the party that information, he won't waste time repeating whatever you said. A huge con is that if you judged it wrong and the player actually wanted to lie about that information or hide it from the party, the rest of the party will have to be way too self-aware about metagaming.
It is a very huge restriction, as most of methods of handling metagame are essentially "hide the information".
Now, if the information needs secrecy:
How often does that happen?
From my experience, it shouldn't happen often enough. This means actually writing a note or leaving the room and talking with the players alone, while taking time, shouldn't make a problem because it happens seldom.
If, for some reason, your campaign is such that you have secret information often (PvP campaigns, notably):
Is it a planned information?
As a way to fasten things up, for your second bullet
A character possesses a specialty or area of refined knowledge such as masonry or architecture.
That piece of architecture is (probably) planned, and you probably know your PCs well enough to know if one (or more) will be able to identify/understand it.
Write the note beforehand and then just hand it over to the players when needed. From what I understood from the question, what takes time is writing the note, not the players reading it.
Finally, if you have lots of secret information, often enough and usually improvised...
How willing are you to play online rather than IRL?
As mentioned in a comment, playing online has the advantage of having private chat rooms. If you use a voice system like Discord, you can also have private voice chat rooms. Note that playing online leads to metagaming problems of its own, for example here and here.
As a possible solution, I usually use my notebook when DM'ing (even IRL), as I have some online tools that I like to use while playing. I find it way faster to open the Messenger or WhatsApp and type the note in the player's chat than writing a paper note. That obviously depends on a) using a notebook during the session b) having every player's contact info c) they having access to their phone or another device to read the note.
If you really don't want to play online...
Well, unfortunately I can't propose a solution that helps you in a situation like that. From my experience, you have trade-offs. You will have to accept that your players will have knowledge and they will have to self-censor the metagame in them, or you will spend a lot of time censoring that information for them.