If you build an index on an abacus containing a Lookup tile, it is considered a ‘fragile’ index.
Because the data used in the index is actually in another relation, updating these indexes when the data changes is not practical. Helix simply marks them as invalid, and they will require rebuilding whenever they are needed. For large relations, this can be very time consuming and have a negative impact on performance.
Generally, it is a bad idea to use a fragile index. It may be better to create a field in the Relation where you had created the Lookup abacus. Then you can either post the data into this field or use the Lookup abacus on the entry form in a rectangle with the field so that the looked-up information is saved in the field when the enter command is executed.