Your problem is almost definitely that you are sending transactions back to the node's own address, and this is inflating the wallet with unspent transaction outputs, causing a long delay in building each next transaction. Instead we recommend starting again, but sending assets to the chain's burn address (available in getinfo).
Another problem will be that each time you send one transaction, you are starting a new Windows process (multichain-cli), which is itself a slow operation. To get a real sense for MultliChain's performance, you should send transactions through the API directly from a single process that keeps running.
