In the 1.0 preview, AdblockIE ran most of the filtering asynchronously. That is, the webpage finished downloading, IE fired the "DocumentComplete" event, and AdblockIE spun off an asynchronous delegate to filter the DOM. This meant that, depending on threading considerations, the page could be loaded for a few milliseconds before it got fully filtered.
In 1.1, I did away with this, for the simple reason that my limited perf tests showed no advantage to this approach.
But with Apple, Google, Mozilla, and Microsoft releasing browser benchmarks every other week, you might be wondering what effect AdblockIE has on browser performance. I did a little bit of testing with the PerfTest BHO that's checked into the AdblockIE source tree. Unfortunately, as is often the case with this sort of thing, the results are far from conclusive.
In most cases, AdblockIE seems to add at most a few hundred milliseconds to total page load time. That's within the same order of magnitude as conscious visual perception, so probably not something to be too worried about.