perf: reduce the BPS tree perftest dataset

Since the performance benchmarks for three additional flavors of the
BPS tree had been introduced, the amount of test in this suite has
increased to 228. Given some tests work with datasets of 10M entries,
the amount of time required to run these increased significantly.

Mitigate this by reducing the test datasets.

NO_DOC=perf test
NO_TEST=perf test
NO_CHANGELOG=perf test