It depends on what it is doing
Below is from Crystal Help...
Key uses of SQL Expression fields
To maintain optimum report processing speeds, avoid using formulas (whether Crystal or Basic syntax) within record selection formulas. Instead, replace the original formula with an equivalent SQL Expression field, and then incorporate the SQL Expression field into your record selection formula. Doing so will greatly improve the chances of your record selection being pushed down to the server.
In addition, avoid sorting, grouping, or totaling on a formula field (whether Crystal or Basic syntax). Instead, replace the original formula field with an equivalent SQL Expression field, and then sort, group, or total on the SQL Expression Field. Again, this will greatly improve the chances of the processing being done on the server.
Finally, if your database supports Case Logic, and your report needs to summarize an If-Then-Else formula calculation, replace the formula with an SQL Expression field. In such cases, SQL Expression fields enable Crystal Reports to perform the report's grouping on the server. For more information, see Using SQL Expression for Case Logic"