Just plug something else in bypass the vm router to your modem and see what speed you get..
Easy enough to do a speed test with your VM... Put something on wan and something on lan and run say iperf server on wan side, and then run a test from lan side.. This will tell the max your VM can handle for throughput.. Mine when not firewall/nat could do like 200ish on that old N40L..
Vs building a new esxi host, I just went hardware and got a sg4860.. Was cheaper then the esxi host I wanted to build ;) hehehe... I then just got a synology nas, and run a few vms on there that I like to play with. If I add the prices of sg4860 and my Nas and the new sg300-28 switch I got as well it still less then the host I "wanted" to build for esxi... And I use way less power now as well... But I do miss the ease of playing with dev versions of pfsense when it was VM.. But now when I have to reboot my nas for something I still have internet - which is way better then when reboot esxi host the whole network was down, even local routing between vlans, etc.