Is there a way to get the memory pool in order of newest first?
I think you have two options:

1) Patch bitcoin-core and make getrawmempoolentry return the mempool transaction sorted after entry_date. There is boost multi index that would be capable to fast sort the mempool transaction (https://github.com/bitcoin/bitcoin/blob/master/src/txmempool.h#L303)

2) You can try to improve your PHP sort speed. IMHO Arsort is not the best choice.