For most CPUs you can do multiple lookups at once and have multiple in flight. This would allow you to use 256 bytes which should be able to stay in lower level

For most CPUs you can do multiple lookups at once and have multiple in flight. This would allow you to use 256 bytes which should be able to stay in lower level cache once you start doing any meaningful number of conversions. So not necessarily enormous and dog slow... but the real test is put it with the rest of your real code and see what happens. If you come to a certain answer it may not stay the same for another real bit of code.