Which approximately doubles the complexity of the verb system. Doubles - because the aspect is NOT a matter of inflection. It's a matter of knowing two separate verbs with the very similar meaning, yet having significantly grammatical function and very specific semantic distinction. For example, you cannot distinguish future tense from present tense if you do not understand the aspect.A significant difficulty though is that you get no help with the vocabulary. I am not sure I ever quite got to grips with the perfective/imperfective thing,
I learned Russian for ten years, and now, after 30 years I barely speak it. So it really does not depend solely on the pace of learining.If you learn a lot in a short period you lose it if you don't use it.
Try Polish then. It has quite a similar script to English. Or Czech - which uses even simpler orthography than Polish.There is also a psychological aspect. If you tell someone something is difficult they will probably find it difficult. Russian is perceived to be difficult because it has a different script.
But I do not think it would make the difference, as using some training you can get used to the Russian script within a few weeks.
Starting young is an advantage, of course, but I do not agree that it's THE differentiation between Russian (or virtually any other Slavic language) and English.English as a foreign language is typically started young and studied over a long period. Complications are taken one at a time and you get a lot of practice. You may start off saying "I do not must" but you will eventually get it right.
I started learning Russian at the age of 10 (approximately) and English - at about 15, so I have a comparison. With Russian we based quite a lot on multiple similarities between Polish and Russian, and primarily the differences had to be explained. For example, we when we encountered a perfective verb in a textbook for the first time, it was enough for teacher to explain "this verb is perfectve", and it was clear for us, it did not require any further explanations at this stage. I think, you might have had a similar impression when your Spanish teacher explained you preterite perfecto referring it to the English present perfect tense - as both of them function in a similar way.
So it's quite probable, that a fully idiomatic native-level proficiency in an educated register of the language has similar level of objective complexity regardless of the language. But who speaks a foreign language with a fully idiomatic native proficiency? Not many people, in fact, and most of the learners stop progressing when they reach the level which is sufficient for their purposes. And this changes the game entirely.
Slavic languages are much closer to "you have to know everything to be able to speak anything" extreme than English. In English it's enough to knowing a handful of words to produce grammatically correct phrases. In Slavic langiages - in general you cannot. To produce correctly even the simplest phrase, you must at least know the grammatical persons, because the infinitive forms generally are not used for this purpose. If you want to include objects - you have to understand at least two cases (with all the complications resulting from inflection patterns), and at least elements of the gender/category system. If you want to include adjectives - the grammatical cases (which have different pattarns for adjectives than for nouns) and genders are a must. Etc. And we even have not even reached the compound statements yet, or tenses. So wherever the asymptote is, the learning curve of Slavic languages is much steeper than in case of English, Spanish or Italian.