attack bitcoin through writing data directly into output's scriptPubKeys In this post, Steve Ellis wrote :  


  "[...]Writing data directly into a output's scriptPubKey is not prunable[...]where as non-prunable outputs will forever create bloat and slowdown the set traversal(e.g. during block verification).[...]


Wouldn't this be a relatively cost effective way to slow down bitcoin by writing the maximum possible data into into scriptPubKey?
No. This is very fast way to spend your money for nothing. You can not put arbitrary data into to scriptPubKey because there is limited number of standard scripts.

The best thing you can do nowadays is sending 550 satoshi (0.00000550 btc) for bare-multisig outputs m-of-3 created from uncompressed public keys. This will enlarge the UTxO database ( look at https://statoshi.info/dashboard/db/unspent-transaction-output-set ) but will not slow down network itself