I have Nectar2 loaded as FX on a lead vocal track. I want Nectar's Harmony module to switch in for choruses and out again for verses. An automation envelope lets me do this but when the project reaches the 'switch on harmony' node the playback momentarily slows, as if the processing of the node is cumbersome or something.  I have the buffers for my RME Aio at 1024 but switching them to 64 makes it neither better nor worse.
 
I thought I might be tackling this all wrong - maybe Nectar should be on the track (Harmony module bypassed) and also on a bus (Harmony module enabled) and for the choruses the track's Output should switch to the bus. But there isn't a way of doing this.
 
I feel I might be missing a simple trick here!

Others will have to comment on your settings, but as far as Nectar 2, I use it that way also without problems.
 
Add Nectar 2 to a bus.
Set Nectar 2 to Solo Harmony Voices.
Insert a Send on the lead vocal track and automate it as desired.
 
From the Nectar 2 bus, output to another bus to add any additional FX, if needed, to process just those Harmony Voices.