SEOUL—When 5G services launched here in April, Jang Dong-gil was among the first wave of South Koreans to sign up.

Now eight months in, Mr. Jang, a 30-year-old tech company worker, has a chilling review for the next-generation technology: 5G hasn’t lived up to the hype.

“I don’t feel the difference,” said Mr. Jang, who uses a 5G-enabled Samsung...