Growth tips ?

Can anyone give me any tips on increasing the growth rate of Lophocereus Schottii Monstrose ?

I obtained a small cutting about 10cm in length approx 2 and a bit years ago. I planted it in my side cacti garden which is extremely good soil, all my other Trichos etc grow like weeds, and is a sunny position with some shade due to other taller cacti and some leafy plants.

During the time I've had it growth has been quite slow and I was wondering if there are any tips to speed things up a bit. I have heard LSM is a picky plant when it comes to soil and light etc. Montrose plants do tend to grow slower but this is very very slow, however it looks healthy enough.
Any thoughts ?

A current picture of the plant is below;

It is just really slow, about 4inches of growth per annum is standard. I have been working to propagate this one for eight years and in that time I have gone from 2 plants to about ten but now the ball is rolling I expect to increase that to around a hundred in another decade or so.

Undoubtedly a spectacular plant, I just wish it was grow a bit more. I believe they will shoot pups from the base eventually.

P.S Just had a look at your grow room/s pictures, so, so awesome. I can only dream.