Usually Vis takes ages if you have not converted enough to func_detail. Usually 90% of my maps are func_detail only the map's boundries and parts essential to block visibility are not func_detail. Take a church like this for example:
http://www.michaeltyler.co.uk/upload...de2-795687.jpg
The pillars, stairs, candles and holders, wall detail etc all would be made func_detail in a map, of course its not a map its an example to illustrate the point...