I've currently got it set up in 640x400 and it seems to slow down when there's alot of action happening on screen

is there a way to make run faster with dosbox?

Control + F12 should speed up DOS Box like a hundred cycles or so each time you press it, which should speed it up (I used this with wing commander) control + F11 slows it down if you happen to make it too quick

Or if you're good enough, set the cpu to use max cycles in the config of dosbox :)
Be aware though that Build games ARE slow. They were the CryEngine back in those years, no kidding.

That Ctrl + F12 seemed to have worked for me

is there a way to keep Dosbox running at 105% without having to always press those 2 buttons? .

Yes, read my post again.