wp-admin performance in multi-site

Hi fellow advanced multi-siters.

Any thoughts on where to investigate a slow wp-admin performance?

Front end rendering is fast, very fast, TTFB in less than 100ms. Bigish server (30gb memory, only 6GB used at any time, 8 CPUs load average at about 0.5 out of 8, SSD )

But in wp-admin, its all pretty slugish.

Places to look? (obvioulsy switching off plugins process of elimination etc, but of course that is tricky in a production environment )