how to optimize a update query

i am using sql server , and try to update a table a single field with a update query ” update table alpha set field1=’23’ where field2=’asd'”

now i have to run this kind of query 70 times/sec .Now in some cases query takes 7 ms to complete , but some time time increase to 200 ms , n i can’t afford that . so can anybody help ..how to reduce query time