You can get stim pack time as loot, it activates as you open the chest, and I don't want to waste it. I actually got a chain of the stuff going into the XI free week, and I had to stop opening boxes so it wouldn't keep going.
Stim packs make leveling 3-5 (and above) not a butts slog.
Also as you learned, you shouldn't spend until you open all your boxes, and strictly speaking, I still have a lot of boxes within reach.
Also I'm waiting on 7k gold heroes to go on sale before I get them with gems, since it maths out to more heroes than getting 10k heroes on sale with gems.
A 7k hero on sale is 312 gems, a 10k is 375. And I have 100 "spare" gems, so that's 8 7k heroes. After that, it becomes 125 levels worth of gems to get 2 10k heroes.
If I used those gems on 10k heroes, I'd only be able to get 4, and I'd still have that "spare" 100 pestering me to "buy more gems".