I don’t have KEF speakers or subs but reading this post (i.e. @aidan_Gaule comments) there seems to be similar issues I’ve had with subs on auto to power up when the signal is detected. In my case, JL Audio support suggested I use a RCA splitter with the left and right sub inputs to double the incoming signal strength. That helped and the subs woke up quickly.
Brief history, have isolated power source for my media closet and entertainment system. With the subs connected to the same power source there wasn’t any noticeable hum in the subs. I rearranged the listening room and had to move the subs to locations in the room where the isolated power was not available and had to use house power.
Using a different power source (ground source) caused noticeable hum and I had to remove the RCA splitter, halving the input signal and bringing back the slow wake up. I started playing a Bass Test recording I found in a Roon search and that woke the subs up quickly but they would eventually go back to sleep if the signal was low enough.
The room correction in my receiver had a sub setup that wanted the sub volume set around 75DB and that kept the hum at an annoying level, enough to hear when the music was quiet. When I turned the subs down to around 65 db the hum was below hearing level. The room correction boosted the signal to the subs to make up the difference and that was enough to resolve the wake up issue.
With the subs at the recommended volume level the incoming signal was usually around – 6 DB. With the subs set below the recommended level the room correction set the signal to + 2 for the front and +10 for the rear. It works, no noticeable hum, the subs wake up immediately and the integrated bass sound is there. I plan to upgrade the living room power with the isolated power from the media room circuit. That will eliminate the power induced hum and I will go back to the standard setup as long as that does not bring back the slow sub wake up.
Bottom line, as Aidan posted, reduce the sub volume level (from 75 DB to 65 DB in my case), increase the music signal level (from -6 to +2 and above) to help with the auto wake up. There should be a sweet spot but it may take trial and error to find it.