Increasing MySQL performance by reducing index size?

I have a table with ~ 1.2m rows in it. It has 6 indexed columns, including one varchar (255) field, which contains URLs.

I need to be able to scan the table to see if the URL exists in the table, hence the index, but I wonder if I will see a performance gain by reducing the size of the index to 50?

Of course, this would mean that you might have to crawl more lines when searching for the URL in the database. But I need to make this request only once every 30 seconds, so I wonder if the smaller index size will be worth it. Thoughts?