Hey William,
The easiest way to import old data is using our LimeLM interface: https://wyday.com/limelm/help/switch-to-limelm-turboactivate/
Of, if you're using the LimeLM api, if you want to generate bulk product keys then generate them using this function call: https://wyday.com/limelm/help/api/limelm.pkey.generate/
Note you only have to call it once and you can generate X keys with Y activation slots. No need to call it 600 times.
If neither of those methods fits your business needs and, for instance, you can make a single call to limelm.pkey.generate, but rather need to make a call it it for each of your old keys, then the best way to handle it is to make the calls in chunks (for example 100 at a time, wait a half a second, then another 100).