Trying to finish the Great Commission "impossibly fast" can lead to failure and abandonment of the project altogether. What is "impossibly fast"? Let's think about the idea of "closure" (e.g. the task is finished) at three levels.
First is the "micro" level - me and my house. "Closure" is complete when everyone in my house has ready access to the Gospel. Generally, if I'm a believer, you could say the house has reached closure because they have me. Obviously there are some households where someone is a secret believer, so we might argue about whether the others actually have access to the Gospel. Most conservatively, let's say that if I'm an open believer, the house has reached closure.
Next, there's my neighborhood - the immediate web of relationships around my household. This could include people who live in the same area, people I work with, people I buy goods from, people who regularly come to provide services. How long would closure take with these people? If I'm an open witness, we could estimate anywhere from a few weeks to a few months to a year. (This doesn't mean they would all become believers: finishing the task does not mean everyone will believe.)
Third, there's the wider area around me. This might include 2nd and 3rd order social connections - perhaps as much as my town or community. My own city of Garland, Texas has a population of about 250,000. By my research (see "a task too big for loners" as an example), this requires at least 2 movement strategy teams, and perhaps as many as 20 to 30 local worker teams (each with ministries covering about 10,000 people!).
This third level of closure requires another order of magnitude of responsibility, thinking, planning, and strategy than either the household or the neighborhood. While my neighborhood might take a year for me to individually reach, this third level requires time to identify the Strategy Teams, and then time to identify, recruit and train the Local Teams, and then time for those Local Worker Teams to implement whatever strategy (perhaps 10 local houses each covering 1,000 people...) to reach "closure."
It's no wonder when considering the third level of closure that planning and execution could take up to 2 to 5 years before any significant movement is seen. It takes time to recruit the workers, not just to evangelize the lost. It takes even more time to disciple converts. ("Not being able to capitalize on widest exposure is the illest of omens"--read this examination of the Peach social network, which this analyst feels is dead, and see the lessons for movements).
If we try to rush an evangelistic program and "mass evangelize" 200,000, we'll probably have to use "brute force" industrial methods. These will not result in an organic movement capable of seeping into the total audience, nor an organically-grown church capable of reaching the next generation. At best, they will result in some converts and simply have to be repeated in a few years time when the next generation is older. At worst, they will be ignored as easily as advertising.
When people try to rush the task, and we don't give enough time to put in place the kind of resources necessary to reach 100,000 or more people, early failures will lead to discouragement and quitting. All sorts of reasons are given. But the simple fact of the matter is, as parents tell their children everywhere: effectively doing a job means taking the time to do it right. Jesus took time to train his disciples - at least one and maybe two before sending them out the first time. Why should we think it won't take time for us?