Thief is relatively slow to startup but once loaded it runs fine. Thief has a benchmark in the menu so we can test video cards for performance. This game requires about 4-5 TFLOPS of graphics performance to play with maximum graphics at 1920×1080 and maintaining 60fps.
See the page on game performance.
COMPARATIVE GPU PERFORMANCE CHART
See the page that features a graph of the relative performance of video cards.