You seem to load the entire table into the data table and then search the data table in your C# code. This is the most horrible thing to do. Instead search data in your SQL code and bring only those rows that match the search criteria.
SELECT Col1, Col2, ....
WHERE Col1 LIKE '%<<searchText>>%'
OR Col2 LIKE '%<<searchText>>%'
Create Indexes on the columns that you are searching. It might help.