You don't, in general, improve the performance of a table-- a table does not have performance characteristics. You improve the performance of SQL statements.
Pick the most important (or a few of the most important) queries against this table. Why are those queries slow? Are they using indexes when they should be scanning the whole table? Are they failing to use indexes that they could be using? Are they aggregating data? If so, could you pre-aggregate the data in materialized views? Do the queries scan a large number of rows in a predictable pattern that would be amenable to partitioning (and do you have the partitioning option available?)?