Many immigrants who are here on permanent visas (green cards) don’t take out citizenship, but most do, and the rate has gone up. According to Pew Research, naturalization rates rose from 62% in 2005 to 67% in 2015.
Eligible immigrants from Vietnam, 86%, and Iran, 85%, had the highest naturalization rates of any group in 2015. Above 80% rates are seen for India, South Korea and a few other countries. The rate among Chinese is 76%.
Mexican immigrants have long had among the lowest U.S. naturalization rates (42%) of any origin group. I bet the higher rate of naturalization is due to more eligible persons being from Asia than from Mexico.
There are about 45 million foreign-born persons in the U.S. 44% of them, or close to 20 million, are naturalized citizens. An estimated. 9.3 million are eligible to apply for U.S. citizenship (that doesn’t mean they will pass the tests). That leaves about 2.6 million legally here but not yet eligible for citizenship. The 11 million illegal immigrants are of course not eligible.
To be eligible for U.S. citizenship, immigrants must be age 18 or older, have resided in the U.S. for at least five years as lawful permanent residents (or three years for those married to a U.S. citizen), and be in good standing with the law, among other requirements. The process to begins with submitting an application and paying a $725 fee. It culminates with an oath of allegiance. Current processing times range from seven months to a year.
The U.S. government denied naturalization applications from 2005 to 2015 to 11% of the 8.5 million applications filed during this time. The standards are here. Ability to speak English is one of them but there are exemptions based on age and length of time in the U.S.