Sharing my fix for the benefit of other users:
I've had a Kobo Aura One since several months after they were first released.
Very happy with it in the beginning. UI responsiveness was a reasonable (not "laptop fast", but under a second in most cases for the interface, and usually less for page turns - not bad for what I regarded as an embedded system similar to an MP3 player).
I almost always use it with WiFi turned off for more battery length, turning it on only for purchasing new books (and occasionally browsing the recommended list). Usually at least a dozen books open for reading and a few dozen unread queued (I'm a voracious reader but often will change books depending on my mood or the book content).
As time went on it became less and less responsive, to the point where it was taking
10 to 20 or more seconds to respond to a touch 
(excepting page turns in an open book which remained about the same) - main menu, menu choices, changing to a different interface area. Soft and hard (factory) resets didn't fix it - maybe some temporary relief for a few minutes. Becoming unusable.
Touches still registered, because the effect of them took place 10 to 20 or more seconds later.
Enlightenment came one day when I was working on a PC - had some software that tried to look for an online update when I had my network cable unplugged and not connected to WiFi. The software slowed down considerably while it made repeated attempts to contact an unavailable online resource.
What are the equivalent online resources on the Kobo that I can modify? The
Wishlist (stored/synced online) and the
Collections folders (likewise).
Cleared both of those out (completely empty, and yes it was initially slow to do this with the sluggish response) and
my Kobo's responsiveness is hugely improved 
. Maybe not quite as per the original experience (sometimes still a few to several seconds to change to Home or My Books or opening the sleep cover), but not too far off it. The main menu and most touch features respond in sub-second timing most of the time.
Read On!