it should work if you edit koreader/frontend/ui/reader/readerview.lua func ReaderView

nSetInv
like this(end of the file) :
function ReaderView

nSetInv(x)
-- print(x)
if x == "inv" then
Screen.inv = true
UIManager:set_FULL_REFRESH_COUNT(DRCOUNTMAX_inopt)
else
Screen.inv = false
UIManager:set_FULL_REFRESH_COUNT(DRCOUNTMAX)
end
end
EDIT: this will not work

maybe change global var DRCOUNTMAX instead