Hi there and welcome
To confirm, are both units slow to access or just one?
What is the make and model of the router it is connected to?
And what have you already tried to do to resolve this?
have you done a full reset of the Linksys yet? So held the reset button in for 10 seconds, release then set it up again?
ok, can you please do this and then setup the Linksys again like when you first received it.
Tell me if this improves the speed please.
if you connect via cable to the linksys, is it faster?
ok, do a repair of the hard drive on the Nas
Power off the mac
Then start the Mac and hold down the Command and R keys at startup.
Hold these keys until the Apple logo appears. After your computer finishes starting up, you should see a desktop with an OS X menu bar and an OS X Utilities window
choose Disk Utility from the Utilities window, or from the Installer menu if you're started from an installation disc.
Click the First Aid tab.
Click the disclosure triangle to the left of the hard drive icon to display the names of your hard disk volumes and partitions.
Select your Nas
Click Repair. Disk Utility checks and repairs the disk.
Let this complete and when it has, tell me if you still have the problem please.