Hmmm... sounds like you have a lot of brushes withs lots of sides intersecting one another and all in the same room.
I have a hunch that those new brushes you added probably touch or intersect the ones you already had in there and thus a lot of new tiny brushes and vis areas are created which would explain the ridiculously high vis compiling time.
A good way to see how your brushes are split in the game is to use the console command:
Please note this only works in single player mode and not in deathmatch mode.
Sometimes, just moving or sizing your brushes so they don't touch one another can avoid a lot of extra brushes. I suggest you read David Hyde's tutorial at Rust on poly count reduction methods.
It's a must read for all mappers. It's a Quake2 tutorial but it applies to SiN just the same.
Another good trick to reduce the splitting of brushes is texture scaling.
For example, if you apply a 64 x 64 texture on a square column that's 64 x 64 in cross-section and 640 units high, the texture itself will cause QBSP to split your brush in 10 wpolys (world polygons) in the game. If you scale the texture by 2, it becomes 128 x 128 and then only splits your brush in 5 wpolys.
The official Sin entities and scripting reference site