The developers let the community know they are aware of the performance issues with the huge raid on Dark Crag last evening:

While we continue to monitor every server and how the new open RvR performance changes are working out we wanted to specifically address Dark Crag. We are aware of the issues players faced on the server during last nights Fortress raid.

It appears the changes we made opened up the fortress to even more people and you've taken it to the next level! We've made some adjustments, however, we won't know how effective they are until another mass raid begins.

Rest assured we'll continue to monitor and improve the issue best we can to ensure all those who wish to participate in these massive battles are able to without issue. Thanks all for your cooperation and keep up the feedback!