memory allocation

we are using cloud server

*this are memory info*

free -h
total used free shared buffers cached
Mem: 15G 15G 197M 194M 121M 14G
-/+ buffers/cache: 926M 14G
Swap: 15G 32M 15G

*this are disk info:*
df -h

Filesystem Size Used Avail Use% Mounted on
/dev/vda1 20G 1.7G 17G 10% /
devtmpfs 7.9G 0 7.9G 0% /dev
tmpfs 7.9G 4.0K 7.9G 1% /dev/shm
tmpfs 7.9G 17M 7.9G 1% /run
tmpfs 7.9G 0 7.9G 0% /sys/fs/cgroup
/dev/mapper/vgzero-lvhome 99G 189M 94G 1% /home
/dev/mapper/vgzero-lvdata 1.2T 75G 1.1T 7% /data
/dev/mapper/vgzero-lvbackup 296G 6.2G 274G 3% /backup
/dev/mapper/vgzero-lvxlog 197G 61M 187G 1% /pg_xlog
/dev/mapper/vgzero-lvarchive 197G 67G 121G 36% /archive

i allocated memory as per following list:
shared_buffers = 2GB (10-30 %)
effective_cache_size =7GB (70-75 %) ---->>(shared_buffers+page cache) for
dedicated server only
work_mem = 128MB (0.3-1 %)
maintenance_work_mem = 512MB (0.5-4 % )
temp_Buffer = 8MB ---->>default is better( setting can
be changed within individual sessions)

checkpoint_segments = 64
checkpoint_completion_target = 0.9
random_page_cost = 3.5
cpu_tuple_cost = 0.05
wal_buffers = 32MB leave this default 3% of shared buffer is better

is it better or do i want to modify any thing

our server is getting too slow again and again

please give me a suggestion