86Box is just accurate enough to run the original unmodified firmware. It does not emulate cache and RAM behavior, so you won't get performance-related accuracy. You are likely to succeed in Spectre-like attacks against an emulated 386, which shouldn't be possible on a real thing.

If you don't care and just want to go faster on a modern x86 CPU, you can replace the CPU emulation part of 86Box with a hypervisor. I tried running a Socket 7 motherboard firmware on Linux's KVM - given the chipset emulation is correct, it works: it amusingly identifies an AMD Zen5 running at 3GHz as "Unknown-S CPU at M60MHz" and boots to a DOS prompt.