Reducing sample size
One method would be to take many subsets of your dataset, i.e.
bootstrapping, build your models, perform cross-validation and calculate the average performance.  This is a good explanation of how the amount of data affects the model outcomes: https://stackoverflow.com/questions/25665017/does-the-dataset-size-influence-a-machine-learning-algorithm

Play around with the size of your subsets until you start getting stable results.