On my Core i3 3220 @3,5 Ghz, GTX650 Ti and 8 GB RAM I find AC running very well most of the time, even with 22-24 AI cars. True, I get the >95% warning almost all the time, but with 50-70 FPS it doesn't really bother me. With 18 AI cars the warning does not appear at all, so in my case 18 cars seems to be the limit - but still I pick 22-24 cars to race against as my PC is apparently able to handle that, it must be a bit sweaty, though
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Only once did I get a massive slowdown, when some AI cars caused a huge collision after Lesmo 2 at Monza; I registered between 5 and 7 FPS then, but I assume it was an optimisation glitch of some sort, soon to be rectified.