The latest kernel is able to index about 500 HTML documents per second on 2GHz PC. Searching is not limited by the document collection size, rather by the hit list size. If documents are ordered by some factor (i.e. PageRank), then the querying is realized in a (rather) constant time. If not, then you can consider query time related to reading 6 bytes per every hit of a hit list.
Indexing performance is smaller than the performance of egothor 2.x. However, the kernel is still able to index several hundred documents per second.
A benchmark was also conducted to test Egothor 1.2.5-RC and Lucene 1.3.
The engine implements a special algorithm that lowers demand for I/O operation during index updating. Other Open Source packages do not offer the same feature and use suboptimal solutions.
The engine may solve queries in a parallel manner since egothor 1.x. We are not aware of any other Open Source package that supports the same feature in its kernel.
The kernel is fully 64bit. There are not any other technical limits.
Benchmarks