Oh, the things I wish I could have gotten to in more detail if I'd had just one more draft...
So, Becq nailed the essential problem to the wall - an 'exchange' is how long it takes every character in a conflict to do one significant thing. However, the scale of consideration differs depending on what people are up to. And with thaumaturgy, you have the potential for the scales to be vastly mismatched - completing any part of a ritual cannot, by any measure, happen as quickly as a dude snap-pointing a gun at you and pulling the trigger.
It's already enough of a stretch to include stuff like picking a lock vs. a gunfight, but luckily (read: totally on purpose) for us, the system is mainly designed to model narrative considerations of time, rather than actual ones.
Let me say this about the Lore prep rule: it refers to game time, not story time (see YS p. 314). Just because Harry has enough Lore for the tracking spell, it doesn't mean that doesn't have to prep the spell - he still draws the circle, prepares his symbolic link, blah dee blah. His player just does not have to engage the game's mechanics to declare he has done this.
But all that prep still takes time in the fiction, right? So, my first question when thaumaturgy during a fight comes up is, "How far are you into prep when the bad guys show up?" Because if you don't even have a circle set up when lead goes flying at your head, you can't even start calling power in.
But if you showed up and got your shit together before the attack happens? Sure, call a bunch of power, take your stress, and have your spell up in a few seconds!
So, what I might suggest as the primary avenue of resolution is making spell prep during combat an involved Challenge using Lore, to give you the permission to engage the normal power-summoning mechanics, and then you can pace everything round by round and not mess up your buddies and their gunfight. IOW, decide that the Lore = insta-prep rule only applies under non-stressful conditions, and adjust accordingly.