Network congestion is indeed a likely cause. Many blame 5G but it uses the exact same frequencies as 4G, and also some more, notably for short range communication, say up to a few hundred feet.

So most of the time 4G and 5G don’t make a difference, you’ll notice the same improvements or worsening which can have a myriad of reasons.

There seems to be an utter lack of enough investment in more bandwidth and throughput in the backbone infrastructure.

Slapping a 5G sticker on a product is not much more than marketing in many use cases. 5G shines in highly urbanised areas, IF they build out better infrastructure that is.

This. Network congestion because of lacking infrastructure is the cause.

Many people are somewhat misunderstandingly blaming 5G. But it uses the exact same frequencies as 4G, and also some more, notably for short range communication, say up to a few hundred feet.

The added frequencies in 5G shine in short distance communication. For example to enjoy high speeds at home or in your car in the city. But you have to have a line of sight to the nearest 5G transceiver. Which in rural situations will never be near enough so you default back to the same ‘4G’ tower.