Huge latency

Hi. OK - some questions: If you create a fresh project and add just one track, set to record and monitor and play your guitar through it. Make sure you have no Reaper FX on the track or the master track or any Monitoring FX set up. Do you still get the latency? Also are you using the very latest ASIO driver for the Apogee unit? What is your ASIO configuration (Reaper, Preferences. Audio, ASIO)?

You need to update to 10.12.5 for USB and BT problems that have been fixed. Also for security, although that won't screw with your sound :D There are firmware updates for a LOT of older machines. These are meant to plug a big and ugly hole in the EFI that allows persistent malware. "Persistent" meaning it doesn't go away after a complete format and reinstall cycle.

When I was running 10.12.4 and I tried to update EFI after downloading the file from Apple, the system told me "You must be running 10.12.5 to install". So I updated to 10.12.5 and tried the installation again. Then it said "You must be running 10.9.5 to install". Confused. Latency still massive and Reaper unusable.

Success! No ideas how or why but it's working now. One track with VST on my bass, latency seems OK. All good with drum VST running too. (EDIT) On reflection, maybe it was something to do with the aggregate device including BT headphones as you suggested. The latency was there previously when other headphones were plugged directly into the iMac, so I assumed the BT cans were not the culprit. However I've been through so many iterations of Audio MIDI Setup that maybe the current configuration (Jam 96k as input + inbuilt output as aggregate device with headphones plugged directly into iMac) is the only configuration that works and it's possibly the first time I've used it. There has to be a logical explanation why it suddenly works and the changes in aggregate device settings in OSX and audio preferences in Reaper must be it. (END EDIT) I need to do some reading on Preferences. Should I tick "Request sample/block size" and specify these values? Where should I be configuring settings with regard to latency? It seems there is a lot about preferences I need to learn to make Reaper run as efficiently as possibly with my hardware and OS. Thanks very much for your assistance.

Thanks serr. You've saved my bacon and provided some great direction to help me expand my knowledge. Much appreciated :)