I'm going to compile the latest build with the test_codec app and test a few aac-files.
EDIT: I'm using the sample from here
nero_096.m4a Decode time - 93.20s 188.79% realtime
nero_128.m4a Decode time - 97.36s 180.73% realtime
nero_192.m4a Decode time - 103.41s 170.15% realtime
nero_256.m4a Decode time - 107.65s 163.45% realtime
nero_320.m4a Decode time - 113.57s 154.93% realtime
nero_400.m4a Decode time - 119.60s 147.12% realtime
nero_q0.3_(he).m4a Decode time - 312.55s 56.31% realtime
nero_q0.2_(he).m4a Decode time - 318.22s 55.31% realtime
nero_q0.1_(hev2).m4a Decode time - 350.45s 50.22% realtime
I don't know, how much CPU-Time PS and SBR respectively need, but the impact of SBR is pretty hard, especially since the average bitrate of the q0.3 file is 86kbps, whereas the 096 file has an average bitrate of 90. q0.4 results in a LC file with an averave bitrate of around 100.
I used the August 2007 build of the Nero AAC Encoder and b16698 of Rockbox with the test_codec-modification.