Android already supports enabling kCFI[0], and says they saw negligable performance and code size impact. Even if it was 5%, security mitigations with a large security impact probably make sense to enable for a lot of usecases.
There is actually study which notes both Anrdoid and Linux Kernel.
On Linux kernel the performance impact was from 2% to up to 25% and size increased around 30%.
On Android there was too much variance, but they note that Google got around 2-3% overhead, which sounds reasonable.
Without hardware acceleration (e.g. Intel CET), it will likely come with great cost. But we are yet to see those benchmarks.
But I would argue that you can take bigger impact for performance on Android or consumer phones anyway, since they are not performing high computation 24/7 usually, and they already have more computation power than most users require.
0: https://source.android.com/docs/security/test/kcfi