A
portal is the area where two
visleafs (each being one or more
BSP leafs) connect and see into each other. The
VIS compiler checks line of sight between the portals, not from the leaf interior to another leaf interior.
A portal file is generated separately by the
BSP compiler on successful BSP partitioning process (i.e. the map has no
leaks.) The VIS compiler uses this portal file as input, and outputs the
Potentially Visible Set (PVS), stored in the BSP leaf data structure.
This relates to wpolys and r-speeds, because if you can see from one leaf/portal into others, then all the wpolys in the other leafs are drawn by the game engine and r-speeds are affected.
For your common gamer: A cutting-edge, extremely humorous game by Valve introducing the portal technique.