SQL Server connection slows down drastically after some time

hi,
we have an sql server 2014 express installed in 4 P.C's and and all the data is coming to 1 PC SQL table. Now every 8 months the connection slows down drastically. What can we do to increase the speed.

How much RAM do you have and what kind of disk?
How often do you do backups
What recovery model are you using
Etc