Thank you for the replies.

It looks like after running slow for a day and users complaining, things became better and faster on day 2, Does it take a day or two to cache plans and save in memory and then use them?

Queries which took 17 seconds on day 1 , ran in 2 seconds on day 2. Memory was bumped up by the IT Guy.

The server has 96 GB ,memory allocated and SQL has been given 81 GB and rest 15 GB to the OS.
Currently the server is using ~88% Memory. If it gets to an alarming level,and we reduce the memory on SQL Server to say 70 GB  to fix it , we will see reduction in memory utilization  but I think SQL Server will again need to cache plans in the memory, so again performance will drop and become good in a day or two. Is that correct?