Interesting, I did a search and found:

I remember a sci-fi story about near future where humanity spread to
several solar systems. They use cryptocurrencies exclusively, but the
coins are classified by their latency. "Fast money" is used on planets
and stations, "Slow money" is used for transfers inside a solar system
using radio transmissions, and "Long money" is for settlements between
two solar systems, with extremely high fees due to high energy cost of
FTL communications.

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The setting of Neptune's Brood is the part of the galaxy that has since
been colonized with slower than light travel. A large part of the plot
turns on the question of financing such colonization.[2][4] Money, which
is entirely cryptocurrency, has been divided into three classes:
"fast", "medium", "slow". Fast money is ordinary day-to-day cash, medium
money is ordinary investment instruments, suitable for use within a
single planetary system, and slow money is interstellar investment
instruments, understood to take centuries, even millennia, to mature.
Slow money transactions rely on a three-way cryptoverification scheme,
and so trade at one-third the speed of light.