First thing is to figure out what is your current bottleneck. I suspect it will be either internet bandwidth, CPU, or hard drive.
If you open up task manager (or even better, resource monitor) you can figure out if your CPU or hard drive is being overloaded.
Internet is a bit harder, the only sensible way I know is to get software that can monitor router using SNMP (easily found for free), but often cheap/domestic grade routers won't support SMNP which makes this method harder. You could look at network utilisation using task manager / resource monitor, but this will be only looking at your server NIC. Just for example one of your kids could be running bittorrent on one of their pc's 24/7 and you would never see this from the server, only from the router.
If it's CPU/RAM, maybe worth considering upgrades - when people starting talking about these microserver's I turned my nose at the idea when I realised how little CPU grunt they had.
If it's hard drive, then maybe time to consider moving the shared stuff to an SSD, or a RAID array (best to consider RAID 1, RAID 5 or RAID 6). If you go down the RAID route, software RAID is shite, and low end RAID cards are normally basically software RAID. Proper hardware accelerated RAID cards aren't cheap (unless you get them 2nd hand). From reading this forum, a few people seem to be big fans of UNRAID. I can't say I know too much about that except it seems to be an operating system as well as a software RAID version (that seems to perform better than regular software RAID). My personal opinion was that if it was that great, big businesses would have ditched their RAID cards to go to UNRAID, but I haven't tried it myself so I shouldn't really try to say which is better.