What is the difference between RandomX light mode and fast mode?

RandomX fast mode uses more memory to mine Monero faster, while light mode saves memory but runs much more slowly. Both follow the same mining rules and produce the same results—the difference is how your device does the work, not which coin you earn.

What the two modes do differently

RandomX is Monero’s mining algorithm: the set of calculations your device performs to help secure the network. Monero has used it since November 30, 2019.

The main difference is whether your device keeps a large collection of prepared data in memory or recreates pieces when it needs them.

• Fast mode keeps a 2,080 MB dataset in RAM, your device’s working memory. A dataset is simply a collection of data used during the calculations.
• Light mode keeps a smaller 256 MB cache and calculates the needed dataset pieces as it goes. Here, “cache” means prepared information used to generate those pieces.

Imagine looking up information in a book. Fast mode keeps the whole reference book open nearby. Light mode keeps a shorter set of notes and reconstructs missing details each time. You save space, but you do extra work.

How much memory you actually need

Those headline figures are not complete memory budgets for a mining program.

Fast mode also uses the 256 MB cache to build its dataset. Whether that cache stays allocated afterward depends on the software. Your operating system, browser, and mining program need memory too.

Each mining thread—an individual stream of calculations—also needs its own working area. Multiple threads can share the large fast-mode dataset, so you do not need another 2,080 MB dataset for every thread.

RandomX also benefits from roughly 2 MB of L3 cache per mining thread. L3 cache is small, very fast memory inside your processor. It is different from both ordinary RAM and RandomX’s 256 MB cache.

That distinction matters: having plenty of RAM does not mean you can add unlimited mining threads efficiently.

What changes for mining performance

Fast mode is the practical choice for regular CPU mining when your device has enough memory. A CPU is your computer’s main processor, and RandomX is designed to favor it.

Light mode avoids the large dataset, but repeatedly rebuilding data makes it much slower. The exact gap depends on your processor, memory, and software, so there is no reliable universal speed multiplier.

Your hashrate, or number of mining attempts per second, is usually much lower in light mode. That generally means a smaller expected mining reward over time, not easier calculations or a special low-memory reward rate.

Light mode is especially useful for software that needs to check mining results without storing the full dataset. It is not usually the best choice for earning XMR.

What this means in your browser

A browser may limit how much memory a program can allocate. It can also add processing overhead, meaning extra work beyond the mining calculations themselves.

Light mode may fit where fast mode cannot, but “fits in memory” does not mean “mines profitably.” It can still heat your device, drain its battery, and use electricity. Do not assume a browser miner exposes this choice—or even mines RandomX directly—without checking its documentation.

The bottom line

Choose fast mode for mining when your hardware and software support it. Light mode trades speed for a smaller memory footprint; it is mainly valuable when conserving memory matters more than mining performance.