We wouldn't charge for a plug-in that wasn't worth buying. Indexing speed is 5% of the problem. The real issue is search speed.
The truth is that searching over an index like that in PHP will take up to 1.5 seconds over 100,000 posts. A million? I'd love to see the results on first-hit times. Is 4 seconds acceptable on a major search? Absolutely not.
Additionally, the pro version isn't tied to PHP's memory allocation. It can hold the entire index in-memory and minimize disk IO (a major bottleneck).
I doubt you run an enterprise-level search (our clients do, and wouldn't have bought it otherwise). WPSearch 2 will likely fit your needs.