I have made some comparisons of triangle counts regarding the addition or exemption of line data (roads, railroad, small rivers). The scenery has 2.5 times the triangles with line data than without (3441205 triangles with line data vs. 1343536 triangles without line data).
On an Pentium M 1.6GHz with an ATI Mobility Radeon 9600 (binary-only x.org driver from ATI) in
- 6-11fps and high load delays with line data
- 10-28fps without line data
This cannot be explained by the general ratio of line data vs. polygonal data in the scenery and also is in notable disproportion to the part in the overall area coverage the line data plays.
Rather the two fgsd screenshots show the differences. The line data introduces additional triangles in the adjacent areas.
Maybe not embedding the streets into the terrain but rather drawing them on top could bring an increase in performance. However, I'm no OpenGL expert an cannot assess the ramifications and complications of such an endeavour.