You have a different definition of speed
While time is obviously valuable for brand managers, startups put a different value on it. For most startups, they are working against a clock that will one day expire. They have a burn rate, and they are working as fast as possible to keep that burn rate from making their bank accounts hit zero. Because of this sense of urgency, startups push hard to get their meetings scheduled, and they are following up the very next day asking when a brand will have a decision.
Just the other day, I had a startup ask if I could pull together a pilot with one of my clients in 48 hours. To put this in perspective, I'm lucky if I can get a meeting scheduled with a client in 48 hours, much less get a test signed off. Brands, on the other hand, are generally working against yearly planning calendars and aligning their holistic marketing plans to match when their retailers change their merchandising shelf sets. They are planning for six to 18 months down the road, not to execute a program next week or even next month.
The fix: There isn't an easy fix for this obvious conflict of speed. However, you can make the best of it. First, ignore the advice to only approach brands during marketing planning season because they are making decisions then. Frankly, it is too late at that point. Educate them throughout the year so you'll be top of mind when they are planning. Likewise, while the big dollars are allocated during planning, more brands are holding back opportunistic dollars to use throughout the year.
In addition, respect brands' timing and don't ask for unreasonable deadlines. You will just get a fast "no" if you do that. Finally, keep in mind that the speed of brand decision making is also somewhat intentional since you are talking about billion-dollar brands. One of my earliest managers summed it up best by saying that "big brands have safety barriers built in to keep you from hitting the wall at 100 mph." (Based on the latest from J.P. Morgan, I wish the same could be said for our big banks!)