You try to use pattern matching, but do not specify the leftmost character.

*F* does not use the ISAM index, whereas F* does. If you cannot modify the search condition, the entire table must be scanned.

You try to use just part of a multi-column key, but do not specify the leftmost column.

If you cannot modify the search condition, create a secondary index with only the columns on which you are searching.

The table is growing quickly and new rows are added to overflow pages.

Use B-tree instead.