I’ve been using a NAS drive as a file server but it starting to get annoyingly slow when restoring client backups on to it.
Any suggestions to something that will do the job better. I need about 1TB of mirrored storage and able to cope with 2-3 people working from it.

HP Microserver’s get regularly recommended over NAS’s… might be a place to start looking

Copied this from a previous post (more about using it as a home media centre but you get the idea):

Hp microserver, when on offer for £100 (cash back deal) –
Setup as media server: It’ll take 4 hd’s up to 3Tb each, stick a graphics card in with a hdmi out and plug it straight into your tv.
Load windows home server2011, run xbmc and remotely control it by a tablet.
key software being
webmin (to control over the web)
serviio (dlna server for audio, video,photo)
subsonic server (alternate audio server great for mobiles/tablet and internet)
samba/nfs/SSH/FTP etc

What network speed does the NAS support and what do the client backups consist of? (if it’s LOTS of small files this will always be slower than a smaller number of large files adding up to the same total file size).
Did it used to be quick enough at the same job?

Sharkbait’s questions +1

Also ..

I need about 1TB of mirrored storage and able to cope with 2-3 people working from it.

What do you mean by working from it? What amount of data? Reading/writing to the NAS all the time or now and then ?

The NAS supports Gigabyte connects as does the rest of the network with the exception of the laptop which is normally connected over WiFi
The HP micro server does it come with any kind of operating system. If not what would you recommend for running windows based programs?

with the exception of the laptop which is normally connected over WiFi

You’re wifi is probably the problem. Try connecting with a network cable and see what happens. Personally I doubt spending ££ on a microserver will make a lot of difference.

* I’m not qualified to tell you the truth though.