You see, the tablet can consume anywhere from 3W to 10W (my estimation) so the default charger would take anywhere from 2 hours to 3 hours to be fully charged. With a 20W charger, assuming the same estimation, it would be 3-5h. As you can see, the difference is far from just 20%. That's what I meant saying it's hard to estimate this.
However, one question really deserves to be answered: what's the maximum charging speed that Eve V accepts? When using a 3rd party charger, will it be able to pull, for example, 50W?