HEAP is table type. If you enable HEAP support, it doesn't speed up the performance unless you use that table type for your data. And if you use that table type, you must know its advantages and drawbacks (which are available at MySQL website, under the manual).
HEAP tables store data to your RAM. So, if your server is rebooted - data is gone. Forever.
But, you can use those tables to store not so crucial data, like sessions and it makes sense as it will be rather fast to read/delete/update sessions when they're stored to RAM instead on HDD.