Hi Christophe,
ABD is more demanding of the network so I strongly reccommend increasing the speed (everything else will benefit too!). Connection all your PC's via a cheap 8 or 16-port unmanaged network switch, for example the HP J9559A, will make a huge difference (I only mention that example as I have used similar units for small networks in the past). Get a 16 port switch and you can connect any network printers via the switch too which makes connecting to them simpler than if they are shared from a particular PC. If you spend a little more you can get an entry level managed switch which could be helpful in the future (trunking ports to give more bandwidth to a server for instance).
What kind of device are your shared files stored on?
Moving all of the preference files is better, they need to be in one place.
Read Site Standards - Colour Tables for the background to MS_DEFCTBL, it is not an absolute control for the colour table.
Excluding the default colour book libraries isn't a problem. As long as you have the the colours you need in your own library you won't miss them. I guess you've figured out colour book creation, but in case you need to know more see MicroStation - Creating a Custom Colour Book
View Attribute delay can be fixed by checking the configuration of BB_DG_ECSCHEMA_CACHE, for the background to this see ABD - BB_DG_ECSCHEMA_CACHE. This is another thing that could be affected by a slow network.
