Is Index rebuild and update statistics mandatory after migrating to new environment

Hi,

we have recently migrated from sql server 2008 R2 to SQl server 2012.

Is it mandatory to rebuild indexes and to run dbcc updateusage after migration.We have done a full backup and restore to the new environment, which means i have moved all my indexes to the new server, then what is the reason simple select statements are taking more time after migration.

Also we didnt set the max server memory configuration for sql server. Our physical machine windows 2012 has more than 200GB of RAM. 

What is the logical reason despite of having plenty of memory sql server processing is very slow.Please explain me the importance of setting max server memory.

Thanks