Hi,
This is a reasonable model of two decoupling capacitors 3.0cm apart on a PC.

This is a very simple test case.
Speed of light is 3x 10
8 metres/second
so 3.0cm in 0.1ns
(propagation in FR4 is going to 0.8 to 0.9 x speed of light)
Can you tell the difference between the transmission line model and the lumped model using low frequency signals?

When I run this simulation I get:

So there is an observable difference at a reasonable frequency.
SensitivityEffect of the length of the transmission line:

Impedance versus Td

Phase versus Td
Sensitivity to Transmission line impedance
Stepping the transmission line impedance 1, 2, 5

Sensitivity to ESLModel

Impedance versus ESL

Phase versus ESL

ObservationA difference can be seen between the lumped model and the transmission line model for two decoupling capacitors.
Given the sensitivity to transmission line impedance, the length of the transmission and the equivalent series inductance of the capacitors.
I am not sure how useful this observation is.
The modelling presented here was two decoupling capacitors, there is typically a large number.
Jay_Diddy_B