bitcoind bulk import private keys to different wallets and update with one rescan
You really should use the newer importmulti RPC.

It lets you import multiple private keys at once (and since 0.18, also supports BIP32 derivation etc), and will do the rescan (if desired) once for all keys simultaneously.

If you have accurate birthday information for your keys (a lower bound on the time when the corresponding addresses might be first used on the network), importmulti will even only rescan the relevant part of the chain.