Can you please tell me if you have heap enabled in your mysql and whether it its playing a positive role on mysql performance ?

Do you mean heap type of table?

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).

I have read MySQL documentation but its still unclear for me whether if its a good idea to enable HEAP tables for forum database.

Can anyone please advice ?

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.

Thanks for info