This morning I lowered the target voltage from 27V to 8V, and the result is much better. Off course I need to open the aperture more (camera is now less sensitive), but in roomlight conditions I am still not with a fully open aperture (at 2.8, and lens goes to 1.4).
This lowers the lag to well below 1 second, which is much more workable.
VR302 was used to adjust the target voltage. So far I can say that the service manual from Ikegami matches perfectly with my camera. VR302 was turned up higher by somebody before (paint marking was broken), but now I am well below the the original position.
Hopefully Hamamatsu will provide the datasheet for the N2606, so I have more of an idea what the grid voltages should be, the dark current and a ballpark target voltage number (this seems to vary from tube to tube, and from what I have read, should be adjusted in relation with dark current).
I wonder is the "gain" adjustment on the 7290A is in fact target voltage adjustment. The 7290A has adjustable/auto gain, but the 7290 also has an internal jumper (J202) for ASC (automatic sensitivity control?) "on" or "off" , and this also affects the target voltage, and it could be that it is this jumper together with potmeter VR302 is brought externally to provide the adjustable/auto gain...