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
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.