There are reports that DELETE from MEMORY tables with indexes are inexcusable slow. One of the reports is BUG#30090 - DELETE from MEMORY table with BTREE primary key extremely slow. Closed as duplicate of BUG#30590. There is another report by Mat Keep and Mikael Ronstrom based on sysbench and oprofile. Since information is rather contradictory and incomplete, it is not yet clean what exactly has to be improved. But it is definitely worth to be analysed.