This is absolutely broken. This enables you to cast four-drops on turn 2 (T1 Mirage Land, T2 basic) or seven-drops on turn 3 (T1 Mirage Land, T2 Mirage Land, T3 basic). In Standard, you could, say, cast a T2 Thunderbreak Regent; in Legacy or Vintage, you could cast a T2 Jace, the Mind Sculptor. In Modern, you could cast a T3 Scapeshift (T1, T2, T3 to get nine lands; sac two and tap GG and sac all seven remaining lands for Valakut and six Mountains).
Well, those examples, with the exception of Scapeshift, show a sped up game flow not necessarily brokenness. Because they are all lands, they aren't restricted by color. Theoretically they could go into any deck and the decks that speed out threats are likely also facing sped up answers. You might get some swings in with hasty creatures, but they are just as likely to be met with resistance. The biggest problems I can think about is probably Landfall and the aforementioned Scapeshift (of course there could be more I am unaware of). I could probably add a line somewhere that prevents sacrifice and targeting from other spells or abilities on the Mirage lands. Landfall might be trickier as it could run rampant but I don't know the older landfall cards that well to be sure.