Some of the ways in which Funcom hopes to improve the performance in these battles is to tone down some particles, stylize the view distance to a shorter setting, and change how some spells and abilities propagate. In the feedback thread to do with this announcement, quite a number of people are upset that the eye-candy they currently enjoy may take a hit for the sake of playability in massive PvP, but it is undeniable that this will be the quickest way to improve the situation at this point.
The collision detection issues are not really elaborated on in the official post, but reading through the feedback thread gives you some idea of the sort of things that are going on -- bugged walls, gaps in walls, for instance -- fairly game-breaking stuff when it comes to defending a keep. We also notice at least one reported case of the first siege time on a guild's keep being set randomly, not at the guild's choosing, putting the keep at serious risk due to real-life commitments keeping guild members from defending at the allotted time.
Perhaps the only good thing in all of this is that Funcom has realized that this is a huge deal for its players, and have let us know that they will do their best to fix as much as they can as soon as possible -- they hope that some of the massive PvP fixes will go in with the next scheduled update on Monday. The beginning of Godager's letter reaffirms the company's pledge to deal with issues like this in an honest and open manner, meaning they will not attempt to sweep problems under the rug -- rather, they will share their plans on how they hope to tackle them, and what sort of a time-frame they will require to get the job done. In this particular case, they are "doing a strong push to address all issues immediately".
If you've had first-hand experience in sieging and massive PvP, we'd be very interested to hear from you in the comments section below. As emphasized in the official response post, only a small percentage of players will have actually been involved in this yet, so let us know how it went for you!