Update README.md (#254)

Author: Brechtpd

packages/loopring_v3/README.md

@@ -53,3 +53,7 @@ make
 * run single test: `npm run test -- transpiled/test/xxx.js`
 * print info logs in tests: `npm run test -- -i`
 * print more detailed debug logs in tests: `npm run test -- -x`
+
+Running all tests takes around 3 hours on a modern PC with a CPU with 4 cores. Creating proofs is computationaly heavy and takes time even when multi-threading is used. Run individual tests when you can.
+
+Verifier/Prover keys are cached in the `keys` folder. When updating the circuits make sure to delete the keys of older circuit versions because this is not automatically detected.