Efficiency Factor is quite simple and it is your average speed divided by your heart rate. However you as you can imagine your speed/pace (it is almost the same thing) depends a lot on whether you run up or downhill so you need to find somewhere in your app something that is called adjusted speed (many apps can do it that they adjust your pace/speed to offset running uphill/downhill). Once you have that number find its value for every km/split of your run starting from 2nd or 3rd kilometer (below that your body is only warming up so reading are useless) until the moment you start your intervals (so if you start at 14km then take all split readings between 2nd up to 14th). It is best to use meters / second for speed. Then for each km you write in excel sheet in one cell average adjusted speed in m/s and the other average heart rate and divide cell. Now do it for number of runs (for all of them make sure you compare readings for the same kms) and you will be able to see how your efficiency changes during your run and over time. It even looks better if you can draw a chart. It will look like where each line is a separate run:

https://image.nostr.build/0ed1f9211e97edcb7ab67bb41cf4419a7a4e3aaaa7120d4c6f6cec2e15286350.jpg