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.

Kind of proving why accuracy is not really that important for the PC world. Most era firmware would have tried to run on as much variety of machines possible.