The current game data may be heavily fragmented on the disk from constant useage. The first suggestion should be a defrag of the game partition, optionally the operating system partition should be examined as well for heavy fragmentation.
If performance is still poor, the game data partition could be moved to a different physical disk. This would require the recommendation that additional hardware should be purchased.
Adding a single disk would be enough to move the data and avoid contention between partitions, but it still isn't the fastest way to move data. A striped disk solution would add performance but not reliability - however reliability is not stated as a requirement. The game data could be moved to a striped disk solution - which would be the most expensive solution