My understanding is that since Current = Charges/Time. If there exists a resistance to the flow of charges, then that must mean the charges slow down, meaning that more time is required to pass through a point. So, the current should then decrease. But, since this opposition to the flow of charges doesn't exist in the ENTIRE circuit, it should really only decrease the current in the resistor, right?
In circuit theory we have Kirchhoff's current law (KCL), which to a physicist is just a statement of conservation of charge:
The algebraic sum of currents in a network of conductors meeting at a point is zero.
(This is different from what you said in comments, "the sum of all the current in the entire circuit must be equal to zero." It applies at any single node in a circuit, not to the circuit as a whole)
This means if we consider a simple circuit with two resistors in series, like this,
then the sum of the currents flowing in to point B must be 0. Or, said another way, the current flowing in to point B must be equal to the current flowing out of point B.
Practically, it means the current flowing through resistor R1 to point B must be equal to the current flowing out of point B through R2.
A similar argument applies at point A. Any current flowing in to R1 from point A must be coming from current flowing out of the source V1.
Ultimately, in a circuit like this with only a single loop, the current in all elements must be equal.
The answers in your other question should tell you the answer to this one. the sum of the resistors in a circuit determine the magnitude of the current, which flows thru all parts of the circuit, The resistor limits the number of charges per second, so yes, if the resistance is greater, the charges are slower, but in all of the circuit, in the resistor they are faster than in the rest.