I have been testing 64 bit DOSBox (both the gs11 and SVN build by Dominus) on a 2013 Haswell i7 iMac with NVIDIA graphics, in High Sierra 10.13.4.
I have tried a range of games and they appear to suffer no perceptible slowdown, including a bunch of Sierra AGI and SCI adventures, Rebel Assault 2, Decent 2, Grand Prix 2, TIE Fighter Collector's CD (SVGA rendering), Conquest of the New World Deluxe (SVGA), Lands of Lore 2, Doom 2, Dark Forces and most of the LucasArts adventure games.
There are three games I have tested where I think there is a performance related issue - Warcraft Orcs and Humans (CD version), Warcraft 2 (CD version) and Space Quest 6. Everything seems to run well within the actual game in WC1 and WC2 except when moving the mouse either very slowly or very fast on the main menus. This causes the music to noticeably slow down and start to lag (regardless of sound configuration). I have no idea why this happens, but I suspect it is performance related because it does not happen in the 32 bit build. I tried running both games in DOS32A but the same thing happens. In Space Quest 6 the same issue occurs in the main menus at the start of the game. Moving the mouse in a certain way seems to slow down the music, and it speeds back up again when the cursor stops moving.
In some games where this sound lag issue occurred, such as Conquest of the New World and Pirates Gold, I found that switching to alternate machines such as svga_paradise or vesa_oldvbe fixed the issue, but none work in WC1 and WC2.
Oh yes, I also had the PC speaker in Vette distort very badly in the intro scene music, but switching machine type to EGA (instead of svga_s3) fixed it. Again, this does not happen in 32 bit.
I am also getting the mouse cursor with the block around it - but never in full screen mode. Since I never play in windowed mode it does not bother me.