Nice write up and findings 
Yes, rather significant. One thing to keep in mind is the type of plug you use is using a hlw8012 which is kind of the less accurate ones around (but probably the cheapest so the preferred one for manufactures). Still the 20% a probably do the rather low update frequency of 10 seconds.
Indeed, that will be actually very signficant. Just think about it the second tasmota sends data to mqtt (for example 100W) and the next second it raises to 1000W just to drop again to 100W before the next data point send.
The question is how much “real time” is real time? From my limited knowledge (didn’t use it in the recent years) tasmota doesn’t allow to change the interval of sensor polling (at least in a easy fashion).
Not sure if there is much left to tweak with tasmota. One thing you definitely could try is change *ware and throw esphome on your plug. You will then have not only the option to report values much more frequent to ha (and more efficient using the native api) but can at the same time influence the update frequency of the sensor
The template posted here (looks like compatible to your device) is using a 3 second update interval but obviously can changed to any value you want:
The esphome site features a little guide how to ota update from tasmota to esphome: