Whether 3G or 4G / LTE can provide faster data communication is the latter, of course, but not the same under all conditions. In places with many people, such as near large event venues, communication speed tends to decrease due to congestion, but this is limited to 4G / LTE, and in some cases 3G is relatively easy to connect.
The reason is simply that the number of smartphones that use only 4G / LTE in recent years has increased. In Japan, au has stopped providing 3G (CDMA2000) services from the iPhone 8/8 Plus released in 2017, and both voice calls and data communications are only compatible with 4G / LTE, depending on the carrier's policy .
Meanwhile, NTT DoCoMo and SoftBank continue to support 3G on the current model iPhone 11. DOCOMO has announced a policy of terminating the 3G service “ FOMA '' in March 2026 and SoftBank will terminate “ W-CDMA '' in January 2024, but as of 2020, both 3G will use 3G with 4G / LTE I can do it.
Therefore, if you use a SIM card other than au and au MVNO, you can select 3G as a data communication line. If the connection to the 4G / LTE base station is difficult or the communication speed is extremely slow due to congestion or other reasons, changing the data communication line to 3G may improve the communication environment.
The procedure is easy, open the screen in the order of "Settings" → "Mobile communication" → "Communication options", tap "Voice call and data" on the screen that appears, and select "3G" on the next screen is OK . 4G is out of service area, but it is an effective method even in the situation where 3G can be used, so if you are using a SIM card from NTT Docomo / Softbank (and MVNO affiliate), there is no loss to remember.
Easy explanation of operation procedure