Changing the back end to SQL Server will generally not make your database
faster. In fact, it may make it slower.
There are some genuine reasons why you might want to change to SQL Server,
e.g. if you have millions of records, hundreds of simultaneous users, the
database must run 24x7, or security is very important.
For performance, though it is probably a matter if sorting out why it is
slow and what needs addressing. You could start with the Performance FAQ
written by Tony Toews:
http://www.granite.ab.ca/access/performancefaq.htm