I asked about the boot time on the PSR Tutorial Forum and someone said that it takes 22 seconds or more for his PSR-S model to boot up. I believe he has several expansion packs installed, so I take it that the boot time is affected by how much data the keyboard has to read and load from expansion memory. I presume this can include things like MIDI song files, individual style files, and other files-- user voices, multi-pads, registrations, and so on-- that you've loaded into the keyboard's internal memory.
Something else I just thought of-- but don't know if it might be a factor-- is whether or not the keyboard has a Wi-Fi or Bluetooth connection that it needs to establish during the boot process.