In fact according to a recent test conducted by OpenSignal (via 9to5Mac), they decided to find out how cellular speeds on iPhones have improved over the years over the course of different models released to date. Based on their findings, the good news is that for the most part with every iPhone release, cellular speeds do seem to be getting faster.
There are some exceptions to this though, such as the iPhone SE released in 2016 that actually had slower speeds compared to the 2014 iPhone 6 and 6 Plus models. There is also the iPhone XR which has slower speeds than the iPhone XS and XS Max, but this is to be expected due to the differences in radio.
Of course when it comes to cellular speeds, there are various things that could affect it. For example it could depend on your cellular reception, where poor coverage could affect speeds. It could also be affected by how many people in the area are using it, and so on, so that’s worth taking into consideration when looking at these figures.