cable.
This I changed from a VGA cable with a VGA/DVI-D converter tacked on the end to a HDMI - DVI-D. I had to detect display settings to get the screen to fit, not sure why that was.
All of a sudden I noticed my computer was slow as hell and games were running at around 5 FPS. But the monitor flicker (which looks like horizontal feint orangey lines going up and down on the screen appears more with games and video apps) was gone (as far as I could tell).
I then decided to try benchmarking my PC. I used passmark to do so. According to passmark my CPU score was running at around 75% of what it should be. My 3D graphics at around 400, under a tenth of what it should be!!!
I changed back to the VGA cable re-ran passmark. My CPU was still the same, My 2D graphics were the same, my 3D graphics score had shot up but was still only just above a 1/2 of what it should be. Obviously I've always had problems with this PC, I always thought it seemed a little slow but never really realised how much worse it was than it should be.
Quite honestly all this seems weird as hell and I have no idea why there should be this discrepancy or where the fault may lie. I can't understand why the flicker would be gone yet my performance would drop so bad. Nor why changing the cable would make any difference to performance at all. Nor why it was so sub-par in the first place.
