I believe I've got the scenario outlined in this SU answer, where my 10/100Mbps router is being overwhelmed by a Gigabit switch and the devices connected to it. This comes into play with frequent website timeouts and the like... Chrome's status bar shows "Resolving host..." far more than it ever did, for example.
Here's my setup:
I have a Gigabit-capable PC (ASUS P5QL/EPU, but only connected at 100Mbps for some reason) and a Gigabit-capable Drobo 5N (which IS connected at 1000Mbps), both connected via Cat5e / Cat6 cables to a recently-installed unmanaged Gigabit switch (TP-Link TL-SG1008D), which then connects via Cat5 to my 10/100Mbps router (an ISP-supplied D-Link DSL6740U). My internet connection is 40Mbps down and 3Mbps up.
I do have the Transmission BitTorrent client running on my Drobo, which obviously generates traffic. I have limited its speeds to either 25kB/s or 40kB/s upload and 1024kB/s or unlimited download, depending on the time of day, although the slowdowns I'm troubleshooting happen all throughout the day.
Since installing the switch, I've noticed a nasty speed degradation compared to when the PC & Drobo were connected to the router directly. I only got the switch to improve file transfer speeds between my PC and the Drobo. Is the only solution to the network slowdown on my PC upgrading to a gigabit router (and appropriate cables), or is there some sort of packet configuration or MTU setting on the PC, Drobo, or router (the switch is unmanaged) that can fix things?
If I do have to upgrade the router, is there any benefit to keeping the PC and Drobo behind the switch, or should they be connected directly to the router? I imagine that retaining the switch could ease the burden on the router when I'm transferring lots of data between the PC and the Drobo, but then I also imagine that having 2 Gigabit-connected devices communicating with a Gigabit router via a single Gigabit line could cause some bottlenecking.