Physics Material slow down the game.

There seems to be confusion between a physics material and a collider.

The physics material affects how bouncy and slippery the coins are. If you don't have one, Unity still has to make up values, so adding physics materials shoudn't give any slowdown at all (well, unless all that bouncing slows the game down.)

The collider is the shape they "count as." In this case, best to probably build a low-poly cylinder to use as a mesh collider. For example, your coins might have 64 sides, to look round. The same size 16-sided mesh might be a good-enough collider.

If the coins were all on the floor and never stacked, you could use a capsule or sphere, locked to only spin one way, with the "real" floor below the one you see (otherwise the coins would float.) But your coins can stack. A sphere collider forces them to stack like marbles. You could make the sphere smaller, but then the edges of your coins would overlap. Capsules are stretched-middle spheres. They can't be shrunk into a coin shape.

A very short box collider will make the edges of the coins catch more, slide against each other funny, and either overlap or have an extra gap (if the box is scaled to so the coins holds the box, or the box holds the coin.) The coins will act like tiles. You could rotate 3 or more boxes into a ninja star, but then the spiky ends would catch funny on other coins.

It would probably also work to arrange maybe 8 thin rectangular boxes into the sides of an octagon. Adjust one so it sticks straight out with the far side where you want it, then copy the parent, rotate 45 degrees, adjust so the corners meet, then copy/spin that 3 more times.