weird performance

in my terrain engine I noticed that there are extreme performance differences only when I change my viewvector. actually I don''t see that there''s an increase in complexity when looking from left to right but still... sometimes fps drops from 60 to 40 frames only when I move the mouse and I can''t see why.. I''m using Delphi 5.... I was getting down to the problem earlier, and I ended up with a simple sky box left but still with the same phenomenom. Does anybody have an idea what this performance increase / decrease could cause. I don''t have the slightest idea what would cause the computer to calculate more only when I change the viewangle by looking at a SKYBOX.... thanx