Hi Balaji,
Except V_T685_CO2 all the views having only two tables. As the standard definition of view, the view doesn't store any data but when you request for data it is fetching the data from tables what it is define, So already tables have a primary key if not having then you can create secondary index for better performance(when the table has more records).
For V_T685_CO2 having a view with five tables. For performance issue rather than using these view you can select the data from one table based on these data select the data from one more table with for all entries and so on.
I hope these will may help to you. If any problem occur let me Know
Regards
Pedda Raju