As detailed in this thread:
It seems that "Open Item Copy In External Editor" is working very slowly. For a 10 second item, at 88.2k sample rate, it takes about 10 seconds for REAPER to copy the item over to RX5. I've tested using WaveLab as the Primary External Editor as well and the result is the same, rather slow.
My REAPER mixer is set to 64-bit fp, RX5 only works at 32-bit fp but I changed the REAPER mixer temporarily to 32-bit fp and that doesn't seem to help. Something else is slowing things up.
It seems that a simple copy of the file should open much quicker. I don't have any plugins inserted on the item, or the track the item is on, or the master channel. Also, my OS and audio disks are all solid state. More specs are in the first few seconds of the video where I show "About This Mac".
I don't think it's a computer hardware issue as RX5 can perform CPU heavy tasks like noise reduction on the same piece of audio in less than REAPER takes to simply copy the item. Here's a video:
Let me know if more info is needed or if anybody else can reproduce on OSX.
For copying short item sections into RX5, the wait time isn't bad. But if I want to bring an entire intro of a song into RX5 for noise reduction, it can take an unreasonable amount of time.