Thanks Rich.
Apparently you're right regarding this Ronar, I indeed refered to th older version.
I'm aware about the common opinion of long lenses posing less issue in small aperture focusing-wise and I was close to purchase f/9 240mm lens some time ago, however now I see that despite of that, I can see me suffering shooting with slow lenses in the environment I used to work.
I'm sure it is not the issue of getting my eyes adjusted to the low ambient lighting - I was there several hours (3-4 hours) and my eyes definitely adjusted to that. Moreover, not being very experienced yet in LF, I used to spend more time under focusing cloth trying to achieve the desired composition and focus then many other, more experienced shooters, so that my eyes have even better chance to adjust to the existing lighting conditions.
In that particular case, I was barely being able to apply movements I wanted because noting their subtle effect on focusing over the entire composition was next to impossible due to dim GG image.
The difference in GG brightness between 90mm/6.8 and 150mm/5.6 was noticeable indeed thus allowing somewhat more comfortable focusing with latter one, however even with this one it wasn't up to the level that wouldn't put a real strain on my eyes, I would define 1500mm/5.6 GG brightness in such situation as probably acceptable, but not more.