In one of the environment OOTB AEM Inbox is taking 8-10 sec to load,there were only 20 items in inbox and with even more count in other environments inbox is loading within a second. Any configuration update required to increase the performance is appreciable.
We have cleared all the failed workflow instances.
That being said, I would suggest purging some of your workflows - you'll get better performance when you keep the number of workflows low. There is a purge tool you can use to help with this 
Hope this will increase the performance of loading the inbox workflows.
Thanks for the update,but I observed in our environment this values are already present, means there is already an AutoWorkflowPurge is running by default in all environments and we are using AEM 6.2
Still the request is taking 7-8 seconds to load inbox list
any other suggestions, also to mention we have monthly tar compaction which we ran today morning.Any alternate suggestion on any particular logs where I can observe what is going wrong will be a great help.