Should we use Entity framework, in a large Web application have a lot of DB transactions & more frequently searching request from multiple user?

Actuall Entity Framework is developed over ADO.NET if we talk about performance. Then ADO.NET if faster than EF. 

if your main goal is performance then use ADO.NET not Entity framework.

you must do first all database and its tables fields must be do separate in small number of chunks that will manage better then your large db.

This is a .net ORM Mapper Framework from Microsoft to help you talking with your Database in an object oriented manner.

A database transaction, by definition, must be atomic, consistent, isolated and durable. Database practitioners often refer to these properties of database transactions using the acronym ACID. Transactions in a database environment have two main purposes:

1. To provide reliable units of work that allow correct recovery from failures and keep a database consistent even in cases of system failure, when execution stops (completely or partially) and many operations upon a database remain uncompleted, with unclear status.
2. To provide isolation between programs accessing a database concurrently. If this isolation is not provided, the program's outcome are possibly erroneous.