Here is another sim where the source is replaced with an opamp with a finite slew rate, and also a finite current-source, current-sink capability. Note the huge increase in rise and fall times at the load end. This is primarily due to the limited current available from the opamp to charge and discharge the coax capacitance.... Also, most opamps cannot supply enough current to drive a resistive load that is as low as 100Ω (when R1=50Ω and R2=50Ω). That is where my comment about "unloading" the opamp came from.