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Native English speaker here. I studied Spanish for a few years in High School, and Japanese for a year, and Esperanto for about 6 months of weekly extracurricular sessions. I managed to learn spoken and written Esperanto about as well as I learned spoken Japanese, in roughly 1/10 the hours spent.
That's not strong evidence that learning it to fluency and communication comfort is 1/10 as hard, but learning the basics and a few thousand words is really quite easy for someone who already knows a romance or germanic language. I'd very much believe 1/2 to 1/3 of the effort required to fluency in a second natural language.
That said, I don't think "ease of learning" is enough. There is no path to a designed language becoming universal. Network effects of language fluency are HUGE - the value to knowing a language is so dependent on who already knows it that there is simply no believable adoption rate for any minor language to become dominant.
My hope is that AR + machine translation get good enough in the next era that it doesn't matter too much. And since the future isn't evenly distributed, the "base" language is likely to be one that's very popular today, I'd bet on English, Mandarin (with simplified alphabet-based writing), or Hindi in that order.
That's not a fair comparison. If you know English + Spanish, you should expect Esperanto to be much easier than Japanese; but similarly, if you know English + Esperanto, you should expect Spanish to be much easier than Japanese. Esperanto is very much more like English or Spanish than it is like Japanese, and it will have been easier for you for that reason completely independent of whether it's more learnable than other Latin-derived languages.