> I can hear the voices of "past performance is no guarantee of future results"
It's more like, "past performance is *not an indicator of* future results". That is to say, the current price of the S&P index, or the performance over the last 5 years, is not in any way related to the results over the next 5 years. They are uncorrelated.
I'll say that again - the market's performance over the last 5 years is completely unrelated to how it will do in the future.
I tested this - I collected data on the value of the US stock market going back to the 1800s. The long-term real performance is nearly always 7%, whether your time frame starts from a record high, from the bottom of a crash, from a period of stagnation. You always get 7%-ish long term returns.
That said, if you invest all your money on Monday and the market crashes 25% on Tuesday, there will serious psychological ramifications given the source of this money. I'd say invest ~$50k per month for the next 10 months. Mathematically you stand to miss out on ~$15k in gains that way, but you also insure yourself against a serious drop. You could spread over a shorter or longer time period based on your own risk tolerance.