is it possible to get mempool transactions as hex using bitcoin-core? getrawmempool returns only TXID (verbose off), what I'm doing is calling getrawtransaction for each transaction ID to get a transaction hex.

Bitcoin core doesn't support batch requests and each RPC call takes 0.06 sec, for ~ 5000 transactions, it takes 5 min which is so expensive. 
can I get mempool transactions as hex instantly?
Bitcoin core doesn't support batch requests


Yes it does. Bitcoin Core uses the follows the JSON-RPC Specification for batching requests. Just use that with a custom client of your choice to interact with the RPC server to get that information. python-bitcoinrpc makes this easy to do.