When you use yallo, you make calls and surf in the Sunrise network, which offers very extensive coverage in Switzerland.
Sunrise operates a nationwide 4G network in Switzerland (over 95% coverage) and provides 99.9% of the population with the best voice and Internet connections. With its "5G for People" initiative, Sunrise continues to focus on ensuring the best possible network operation. Sunrise also now plans to install "Fiber optic over air" systems in order to provide better coverage in rural areas especially.
If you experience a slow connection, please check the network coverage at your location and perform a speed test (for example, www.speedtest.net).