When you have it, you won't use it.
When you are using it, you don't know how you are using it.
When you don't have it, you wish you had more.
For some people it will run, for others it lags.
What is it?
I think it's:
Time
When you have it, you won't use it.
Wasting time
When you are using it, you don't know how you are using it.
You use time doing anything
When you don't have it, you wish you had more.
I wish I had more time
For some people it will run, for others it lags.
Time can run or lag
It could be
money
When you have it, you won't use it.
Misers accumulate money for its own sake, with no intention of spending more than the bare minimum needed for survival. Prudent people save money for future expenses: retirement, children's education, and the unexpected ("rainy day fund").
When you are using it, you don't know how you are using it.
People look at their credit card bills and say "I spent how much?", because, while you're spending it, it's easy to lose track of how much you're spending,
When you don't have it, you wish you had more.
Needs no explanation.
For some people it will run, for others it lags.
OK, this one doesn't work so well.