Updated OP 4th Jan
Gran Turismo 5 suffers from a serious online bug which results in abnormally long lap times for those affected.
The bug manifests itself by way of the competitors vehicle having reduced grip, poor acceleration and poor braking. It is usually associated with a noticeable drop in frame rate or stuttering. I terms of measuring the impact for those affected by this bug race lap times can increase around 3 5 seconds per lap (and in some cases by much more).
The exact cause of the bug is unknown. However there are several factors which can increase the risk of the bug rearing its ugly head;
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Room Size
Theres no magic number but rooms of 12+ competitors seem to be a common factor.
Premium Cars
All premium (highly detailed) cars also seem to greatly increase the risk of the bug.
Race Only
Free run mode seems unaffected. Those suffering the bug will not be able to get close to matching their qualifying (free-run) times in the race.
Use of a FFB wheel
There seems to be a strong correlation between using a FFB wheel and suffering from the bug. I can personally confirm this to be the case as well as GTP members;
jon-
https://www.gtplanet.net/forum/showthread.php?p=6433289#post6433289
Speedy6543 -
https://www.gtplanet.net/forum/showthread.php?p=6435703#post6435703