I’ve often had this in the Scripting Editor in Terminal & Editor, but now I know how to make it happen in Renoise itself.
Pattern Editor.
If you hold down right-shift (this is without it being tied to anything, btw) for 3-5 seconds, the keyrate gets shot down to one toggle per one key input. If you cmd-tab away from here and cmd-tab back again, you are then at your regular speed.
This is really rather annoying! 
Could be related to how you can configure the script to key repeats.
There were problems in the default keyrepeat behavior for scripts when various scripts were developed and tested, so you can define your scripts how to respond to these:
– key_handler_func is optional. When defined, it should point to a function
– with the signature noted below. “key” is a table with the fields:
– > key = {
– > name, – name of the key, like ‘esc’ or ‘a’ - always valid
– > modifiers, – modifier states. ‘shift + control’ - always valid
– > character, – character representation of the key or nil
– > note, – virtual keyboard piano key value (starting from 0) or nil
– > repeated, – true when the key is soft repeated (hold down)
– > }
See http://www.renoise.com/board/index.php?/topic/30404-fixed-osx-scripting-terminaleditor-may-lose-keyrepeat/ please. Will be fixed in the next Renoise update…
Wonderful, wonderful, wonderful news. I had to switch to using Coda + TextEdit to be able to handle the lack of keyrepeat in Renoise Scripting Editor 