86Box is far more focused on being as accurate to real hardware as possible, allowing all kinds of old software to run, even the hard ones like OS/2 with its heavy uses of ring 1 and 2 security contexts that are usually entirely ignored by OSes like Windows and Linux (or, well, DOS, where literally everything is ring 0...). It can even run the 8088MPH demo made for the original PC :-)
With this shift in development focus, it pretty much necessarily sacrifices performance for the goal. They are often incompatible goals. See how emulators like Nesticle could run just a handful of NES games on a 486 compared to later developments like Nestopia that demanded a Pentium 4 to run full speed, but do run every NES game ever made.
Citation needed. I played with PcEm but booting a Slackware bootdisk was challenging, to say the least.
Exempting serious bugs (which have become rare), Slackware should install the same way as a real PC with the hardware configuration you select.
DOSBox is meant to be a lightweight DOS runtime on top of a host operating system with the minimal hardware emulation necessary to accomplish that. Generally it makes running games easier as you don't need to deal with autoexec.bat, config.sys, and all the memory management that comes with real DOS. Under 86Box, your VM is yours to meld in exactly the same way as a real old PC; if you run MS-DOS, that means all the nasty parts come back. (I personally recommend installing Windows 95 at least, running DOS games under 95 tends to be a big relief for these same reasons.)