I know you're joking, but that operation is often disk-io bound, not cpu or memory bound... and the disk is still EBS, you'll not get much better performance on that than on an m5.xlarge I bet, unless you use a grep regex that needs to gobble lots of data into memory as part of the search.
With that much memory, a ramdisk would seem completely reasonable.
tmpfs would solve the disk-io
> that operation is often disk-io bound
the first time.. 12TB can cache a lot of logs.
It's disk-bound the first time you run grep, but the second time you run it the log file is going to be in memory, if you have enough memory.