Postgresql isn't using all the memory

Im trying to evaluate Postgresql as a data storage for a time series system that i need to develop.

The postgresql is with this settings generated by pgtune:

• default_statistics_target = 100


• maintenance_work_mem = 960MB


• constraint_exclusion = on


• checkpoint_completion_target = 0.9


• effective_cache_size = 5632MB


• work_mem = 192MB


• wal_buffers = 32MB


• checkpoint_segments = 64


• shared_buffers = 1920MB


• max_connections = 20

I'm using Postgresql 9.4 in a i5 laptop with 8gb. The test data is a table with 100m rows.

The queries are very slow and i have plenty of free space on memory, but postgres isn't using the memory, don't know why.