So, to enable 64bit builds for Android you should only need to enable separate builds per CPU architecture and include the "armv64-v8a" architecture. React Native 0.59 includes a JSC upgrade, which should support that feature.
|include "armeabi-v7a", "x86", "arm64-v8a", "x86-64"|
The result: on the 32 bit build, the operations took 2.4 seconds on average. The 64 bit builds lead to an average of 1.8 seconds. I know that this would not hold as a scientific proof, but anyway. The 64 bit version was ~25% faster!