DOSBox has run pretty flawlessly for me in every game I've tried, bar one and only in a specific situation. Hired Guns screen-wipe effect. For some reason, when the game does its screen-wipe/text-clear effect on the menu, DOSBox slows right down to the point where the audio stutters.

Here's a video of it happening - https://www.youtube.com/watch?v=rTJWe7Jb9T4
(ignore the fact I'm using a shader as I forgot to turn it off - it happens without using shaders as long as I'm in openglnb mode).

Switching to "surface" instead doesn't suffer from this problem and it appears to render fullscreen.

DOSBox was set to Max Cycles, CPU usage never goes over 5% total (all cores combined) and Nvidia's GPU monitor shows GPU load never exceeds 30%, typically 8-11%. CPU is an i7-5775C and GPU is an Nvidia GTX 1060, so the machine's not weak in any way.

Any ideas why the OpenGL renderer struggles so much with that particular effect?

EDIT: Sorry, just noticed there's a Games/Apps specific sub-forum. Can this be moved if someone notices it, thanks!