This blog entry (https://blog.mangolanguages.com/are-some-languages-harder-to-learn) by Mango (an online language learning app) discussed different difficulty levels of new languages for English speakers. The article makes a reference to the Foreign Service Institute (FSI) language table, which is one of the most frequently cited information on the expected lengths to master different languages (for English monolingual speakers — the chart may not work for bilingual and native speakers of a language other than English).

https://www.state.gov/foreign-language-training/

In one version of the FSI tables, several languages are marked with asterisks, indicating it is particularly difficult in the same category.