@quinncain
The thing to keep in mind is that PowerApps is exactly like Excel in that it has to evaluate formulas all down the chain. It is not like a development platform where things only happen when you code them to do so.
So normally, having a Gallery and then using the .Selected can be just fine. But if you have many other dependent formulas that base their values off of the gallery, or the form or anything else that stems from the top (the gallery in this case), then those all need to evaluate. Again, this is, in many cases, not an issue. But if there is a lot, then it can bog down the app.
You can start by replacing all the .Selected references to a variable. Changing the OnSelect of the Gallery to set a variable (i.e. Set(glbCurrentRecord, ThisItem) ) and then setting the Item property of the Form to glbCurrentRecord
This will cause the downstream formulas to only evaluate with data when something is actually selected. Otherwise they will be working with blank data - which should not take any time.
Also, the default settings for the features in PowerApps are typically on the side of higher performance. So, if you have changed any of those, then there is the potential that one of them is set wrong.