I recently upgraded to a new Late 2015 i7 Retina iMac (8GB, 500GB SSD, 4GB m395x video) and also bought Lightroom 6, and a new 4TB Seagate USB 3.0 "Backup Plus Fast" external hard drive.
The external drive is a single, portable, raided device that offers great speeds, about 4x faster than most external USB drives. I get a 240 MB/s read and write bench and a 1GB file can copy to my drive in a few seconds which beats my WD My Passport Ultra 2TB USB 3 drive by a fair margin (about 4x faster as mentioned).
I have all my pics and LR catalogue on the Seagate drive, and I notice when LR is open, the external drive's speed drops by about 75% (about 50 MB/s reads/writes), even if there is nothing apparently going on in LR. I check Activity Monitor and it shows no CPU usage or disk reads or writes. Even my WD external drive will drop in speeds from about 80 MB/s to about 30 MB/s.
A file transfer is also about 4x slower for each drive. A 1GB file transfer now would take 10-15 seconds on the Seagate.
When I close LR, this persists for up to 5 minutes after, the slow speeds persist, then they return back to normal. This is repeatable.
I've tried running only 1 drive plugged into the USB on the iMac, same issue though. What could LR possibly be doing to have this happen?! I initially thought it was reading catalogues or something but again, the Disk Activity in Activity Monitor is basically nil. I'm at a complete loss!
Btw, I have Photoshop CC and it's fine. I can open 1GB PSD files from the same Seagate drive, and no issue at all.