Devices with older hardware or not old, but less powerful hardware. For example, a device with 1GB RAM (this is memory, but is different than storage which people often confuse as memory) and a big island with 800 dragons will probably struggle because there are so many animations and sparkles and stuff happening.
The 2.8+ updates have optimizations that will try to compensate for the device's hardware by reducing animation/sparkles. It's possible there will be a little bit of temporary lag if you have a higher end device after updating if you have a lot of Dragons. For people who are updating to 2.9 after not previously updating to 2.8, it may be more noticeable because they're just now installing the optimizations.