Those are the more or less "global" issues. PC's can handle thousands of packets per second easily but PLC's are much more limited in their ability to process both legitimate and unwanted traffic. I'd go after the source of all that traffic and start working there. If you already have say 500 packets per second worth of broadcast traffic then optimizing your PLC communications isn't going to do much good.
The key here is not to look so much at the traffic itself (wireshark doesn't understand PLC-5 Ethernet) so much as it is to look at traffic OTHER than that intended for the PLC or PC such as broadcast traffic. This will allow you to monitor packet rates, data, etc.
Now plug the remaining free port on the laptop into the mirror port and run wireshark to monitor this port. Mirror one of the PLC or PC ports that you want to inspect. Connect to the managed switch and program the remaining free port for port mirroring mode. Plug the laptop into one of the remaining ports. Plug the managed switch in between say a PC and a switch or a PLC and a switch. Get a copy of Wireshark (free, google it) and a second NIC card (Ethernet port) for a laptop. If you aren't truly sure what's going on, the best thing to do is to buy a managed switch (yes, even a cheap one is expensive), the smaller the better. Sometimes bad network setups such as using Ethernet/IP without an IGMP agent is the kiss of death (PLC-5's don't use Ethernet/IP). You also should be looking at what other traffic is running on your network. Not saying that any of this is your issue but start here. These days it makes sense to run 100 Mbps minimum between PC's and switches, and backbones are now typically running 1 Gbps (even though this is overkill for most controls networks). In other words, if you have all 10 Mbps connections between PC's and switches, then you will be limited to 10 Mbps total even if the PC's can handle a lot more. Also look at your PC's and network switches too and make sure you don't have any ugly situations in terms of aggregate bandwidth.
If your PLC-5's will do full duplex (most of the older hardware won't), then the bandwidth is really 20 Mbps, 10 Mbps uplink and 10Mbps downlink. If you have switches and they are capable of full duplex operation, you've got 10 Mbps bandwidth for EACH device (not total). If you use hubs, EVEN if you optimize your PLC communication, you will still have bandwidth issues. So you've got 3 Mbps total input and output with hubs for ALL connected devices. Within that domain, the total bandwidth is going to be effectively 1/3 of the slowest device, usually 10 Mbps for PLC-5's. If you have hubs, all the hubs that are directly connected together form a broadcast domain. Are you running hubs or switches? With the price difference being what it is, remove all hubs and replace with switches.
First, you've got to consider your basic network implementation. It should be reasonably possible to scale to many more PC's and PLC's even with RS-View32. I have dozens of PC's polling about 2 dozen PLC's.