HotHardware has published some disturbing benchmarks comparing the performance of Android 10 and Android 11 on Pixel smartphones. Only a few devices have received Android 11 so far apart from Pixels, such as the OnePlus 8 and OnePlus 8 Pro. Others, like the Pixel 5, are launching on Android 11, making comparisons hard.
Nonetheless, HotHardware has demonstrated in multiple benchmarks that the Pixel 4a and Pixel 4 XL perform better on Android 10 than they do on Android 11. The Pixel 4a fared slightly better the photo editing portion of Futuremark PCMark on Android 11 than on Android 10, but this is as good as it gets for the two Pixel devices. HotHardware recorded performance drops varying between 3% and 33% depending on the benchmark.
HotHardware stresses that it reproduced these results after factory resetting both devices. Additionally, the website claims to have performed all benchmarks with the latest app, OS and security updates installed. There have been similar complaints on Google support forums, Reddit and within the comments section of HotHardware's article too.
There is a chance that these complaints of slowdowns are all isolated cases, of course. However, the fact that HotHardware has been able to reproduce these performance differences suggests that something more is at play than rogue software.