The Redmi phones not Redmi Note, not Redmi K, just the base series is the baseline for Xiaomi phones, a distillation of the most important features wrapped up in an affordable package.
By their affordable nature, they move slightly behind the times, but the Redmi 9 is still a major upgrade over what came before it. For starters, the screen now has 1080p+ resolution, no more slumming it with 720p+. Also, the diagonal was adjusted to 6.53 (up from 6.22) to make better use of the higher pixel count.
Powerful chipsets have proven their ability to draw in potential buyers, so the Redmi ditches the entry-level Snapdragon in favor of a MediaTek Helio G80. The G-series is gaming-oriented and while the Mali-G52 MC2 is hardly a beast, it still has more than twice the computing power of the old Adreno 505.
The CPU has two Cortex-A75 cores, which deliver a significant boost in single-core performance (the small A55 cores got a speed boost too).
Note: real world performance gains will be smaller, but still notable
Xiaomi kept the memory setup the same as before 3GB of RAM and 32GB storage as base, 4/64GB as an option. Its a standard eMMC 5.1 storage were looking at here and theres a dedicated microSD card slot for expanding it.
The Redmi 8 technically had a dual camera, but since the second module was a depth sensor, you really only had one cam. The Redmi 9 corrects that with a quad camera setup (and yes, one of the modules is still a depth sensor).
The main camera has a 13MP sensor, which feels like a downgrade. It has 1.1 µm pixels and sits behind a relatively dark lens with an f/2.2 aperture compare that to the 12MP sensor on the old phone with 1.4µm pixels, Dual Pixel AF and f/1.8 aperture.
However, the new phone packs an 8MP ultrawide (118°) camera as well as a 5MP macro camera, offering more flexibility. Video recording is still capped at 1080p@30fps, theres isnt even a 60fps option (even though the chipset supports it).
The battery has essentially the same capacity as before, 5,020 mAh. It supports 18W charging, but the retail package includes a regular 10W charger, so if you want fast charging, youll have to buy a separate power brick. This was the situation with the previous model too, so no improvement here.
The Redmi 9 is priced at 150/$140 (for the 3/32GB model) while the Redmi 8 started at $112 or so. Are the more powerful chipset and higher res screen worth it? Or are you sad to see a solid entry-level device get more expensive without a good reason?