If you are okay with the cards being very mildly damaged, then put them on the largest surface area you have available and uhh do the children-shuffle thing... somehow it's hard to describe the technique. Which is actually the most effective way to shuffle cards.
If they are valuable, like MTG cards, well then you have to divide them into a lot of piles, shuffle them individually, and then merge them. That would take a lot longer.
I think it is called the scramble shuffle? Or some people call it the smooshing method.
Nah, the smooshing method/mash shuffle is similar to a riffle shuffle but without the bending. It also requires sleeved cards.
The reality is you need to separate those 1500 into 15 stacks of 100 or so, shuffle each stack thoroughly, move half of each stack to the next stack, shuffle thoroughly, repeat, um, 13 more times, then stack the piles together.
Or get a casino shuffler, though even those likely can't do more than ~500.
Can I ask what sort of game this is? Or are you creating a Battle of Wits deck?