I installed opnsense on an i5-6500T (native, no vm) but it looks like the performance is very bad, most websites are timing out.
everything is turned off, there's no packet inspection or blocks, even the unbound dns server is not used (using a DC for that)
it's the computer that's underpowered, or i did some mistake in configuring it? Using two routers in the networks for my convenience
Your diagram is weird. Isn't the opnsense box supposed to replace the router. Or at least it should be between the existing router and you clients. Pc 1 go to opnsense Lan. And opnsense wan to the router and internet.
You're creating all kinds of loops which is generally a bad idea. Your data should flow in 1 direction like a tree.
Unless there are a lot of details you're not sharing.
Also remember generally a router is not a switch. Plug all your PCs into a switch. Plug a wifi access point into the switch. And then have the switch go into the lan of your opnsense.
And then have the wan go out to the internet.
i had the idea that two gateways could work in the same network without issues... in my inexperience i tried it with three hosts on an hyperv virtual network and it worked.
my stupid idea it's like this:
why i do this? To have a "temporary" setup where i slowly move all the static ip addresses to opnsense and in this way everything can have a valid configuration
You can still do this but as others have said you need to have 2 separate lans. Your old Lan can go to PC 1 from old router. Then opnsense wan goes to your existing Lan and importantly you are now creating a new Lan on the lan side of opnsense. Here you can connect the PC 2 to test with. Each PC should only be on 1 Lan and each Lan should have a separate subnet.
See this post and the last comment even references a diagram to exactly what you want: https://forum.opnsense.org/index.php?topic=32774.0
There are all kinds of routing protocols and algorithms at play which don't like loops and multiple routers competing to control the same subnet.