As Paul says, it's unlikely the EOS M wasn't really designed for the 400mm although it can be fitted to it with the adaptor. There has to be a reason the 1Dx has the extra power compared to even the 5D3. What I understand, it doesn't only help increase the fps it also helps focusing faster. So, the EOS M isn't likely to have that extra power to drive its motor very quickly. I have used a (borrowed) M now a couple of days with the 22mm and that touch screen focus and obviously it isn't fast, but not as bad as I expected. It's all about having a balanced system and have the right expectations.
True. I must admit I am interested by these small cameras, and often find myself at a Sony area of the shopping mall messing around with the RX 1 and RX 100. It will be interesting to see what the new M is like, but for now, if I were purchasing such an item I would get the RX 100.