A big player in becoming a US citizen? Your ZIP code.

This article has been updated to include a statement from U.S. Citizenship and Immigration Services.

The ties are straightened, hair combed, and jewelry gleaming here, and the line to get into the federal courthouse is spilling outside into a chilly morning.

Get here 30 minutes early, someone says; they weren’t kidding. Two U.S. airmen from nearby Goodfellow Air Force Base, here to sing patriotic songs, joke about being late for their own gig.

It’s a long drive from here to San Antonio – 220 miles, about the same as from New York to Washington. But for the 33 newly minted American citizens, all the round trips over the past year feel more than worth it.

Milagros Carnes, from the Philippines, says her American daughter worried she (Milagros) would be deported if she didn’t become a citizen. Alejandro Fraire has an American wife and three American children, and his green card was close to expiring. For both of them, naturalizing just made sense.

“It’s a hard and long process, but it’s worth it,” says Mr. Fraire, a Mexican national who has lived in the United States for two decades.

Barriers to becoming a U.S. citizen can vary significantly depending on where in the country a person lives, according to a recent report from Boundless, a Seattle-based startup that helps families navigate the immigration system.

Analyzing data from U.S. Citizenship and Immigration Services (USCIS), the report used processing times, backlog numbers, and distances to agency field offices to rank the best and worst metro areas for becoming a citizen.

The report doesn’t explore why these geographic disparities may exist, and it includes some red herrings, according to experts. Austin, Texas, for example, is ranked as the hardest city in which to become a citizen in large part because applicants have to report to a USCIS office in Houston, three hours away. But the report also notes broader trends in how barriers to American citizenship have gotten steeper over time.

The volume of citizenship applications fluctuates from year to year, but the past two years have seen a surge in applications. There hasn’t been a corresponding increase in the processing rate, experts say, exacerbating a backlog that had already doubled during the Obama administration. The success of field offices in clearing this backlog differs. In Providence, Rhode Island, 81 percent of those who applied in the past year or had applications pending have been processed, while in Miami and Dallas the number is only 30 percent, Boundless found.

"Despite a record and unprecedented application surge workload, USCIS is completing more citizenship applications, more efficiently and effectively—outperforming itself as an agency. USCIS strives to adjudicate all applications, petitions, and requests as effectively and efficiently as possible in accordance with all applicable laws, policies, and regulations," said USCIS spokesperson Jessica Collins in a statement.

Staffing levels and office cultures and personalities might affect approval rates and backlog sizes at particular offices. Different field offices can also vary on discretionary aspects of the naturalization process, such as an officer determining if someone is of “good moral character” or if the person's ability with English is good enough. Having a criminal history, tax debts, or a spotty record of making child support payments are some indications of character issues that can be interpreted differently depending on the USCIS office or officer.

“The only time I’ve seen variation is when there is discretion on an issue of character,” says Marisol Perez, an immigration attorney in San Antonio.

“I think all the offices follow the law,” she adds. “It’s case by case and officer by officer with regard to [who they say] deserves favorable discretion.”

As part of its efforts to continue eliminating the backlog, Ms. Collins added, "USCIS is in the process of realigning our regional, district and field offices to streamline our management structures, balance resources, and improve overall mission performance and service delivery."

The realignment, effective October 2019, will realign the 88 existing field offices under 16 district offices (instead of the current 24) in order to evenly distribute workloads and provide more consistent processing times. The last such realignment occurred in 2006.

The Trump administration has increased the burden on USCIS offices. In 2017 the administration moved to expand in-person interview requirements for certain permanent residency applications and a year later expanded the interview requirement for married couples applying for a green card. While these changes didn’t affect the naturalization process directly, they have had an indirect impact by increasing the workload on USCIS officers, experts say.

“Placing a lot of emphasis on national security is applying more scrutiny to applications, is increasing their workload in looking at applications,” says Julia Gelatt, a senior policy analyst at the Migration Policy Institute, “and there are proposed changes that could increase that further.”

The administration is considering other changes for this year, including a wholesale fee review and more expansive questions and travel records for citizenship applications. The administration is also reportedly considering shuttering all 21 international USCIS offices, which could slow the processing of family visa applications, with The Washington Post reporting that officials are preparing to do it.

Perhaps the biggest effect the administration has had on the naturalization process has been through its consistent anti-immigrant tone, however.

The USCIS field office in San Antonio saw a 40 percent increase in citizenship applications last year, according to Ms. Perez. The agency changed its mission statement last year – removing the words “nation of immigrants” and adding “protecting Americans, securing the homeland” – causing clients to come to her seeking naturalization.

“They fear their status is at risk because of the tone of this administration,” she says. “These are folks who should have no reason to be worried but are worried.”

“I have faith in the system, and I have faith in USCIS,” she adds. “We have good relationships with USCIS, but that’s the tone we have out there.”