Keg, has largely answered the OP's question.
The RTA can be used, but if it starts chasing ridiculous speeds then I tend to note the speeds it was going to use and program the FMC with those speeds and see how we go.
As a rule of thumb, if the time to be lost is <3mins then I generally try to do it via reducing my descent speed only. When the time to be lost is between 3-5mins then I will also reduce my cruise speed and well as my descent speed.
RENURPP highlights an interesting conundrum, which is, is it better to descend to be able to fly at a reduced IAS>TAS>G/S, but with a higher fuel burn, or should you continue at altitude maybe changing your cost index to zero, and accepting the delay in the holding pattern, but "probably" burning less fuel overall?
As I see it, if you want to fly most efficiently (in this case burn the least amount of fuel), given that the time at the fix is not ours to modify anymore, then we are effectively flying for maximum endurance.
Given the relationship between speed, drag, altitude and jet engine efficiency, I would be curious to know what altitude, speed and thrust setting would be best for your typical passenger jet.
Any thoughts?