I have a two stages scenario to enhance the performance:
1- You told cardinality of "client_id" is low, therefor search for an id results more than 5 percent of all records. this means that index on "client_id" is not suitable. Therefore I recommend to drop the index.
2- You may use "list partitioning" to partition the table based on "disabled" column. This help strongly you to search only "N" or "Y" records. It is helpful if you check partition creating on "client_id", because the cardinality is low. I guest partition creating on "client_id" is more helpful than on "disabled".