Christiaan
Most likely it is not the cells slowing things down it is the clipping of the various pieces of the cells against the corridors.
What then happens is every time a change is made to the corridor or a cell along the corridor, the corridor reprocesses all the individual clips. this will bog down the design.
How many cells have you placed?...
When a cell is used to clip a corridor, multiple clips occur with the cells constructed in the example workspace. IE in the basic T intersection 5 clips- 1- interior terrain, 4 linear templates.
To avoid multiple clips I will now create my cells with a finish terrain surface. The cell is then placed and to clip the corridor the finish terrain boundary is selected ..IE ONE clip.
so with this process I can now place 5 intersections and only use the resources needed for 1 of the sample T intersections.
I will follow up with a youtube video later this week to demonstrate this process.
HTH