A number of the DOSBOX configurations i've noticed love to use stretching which sometimes can do a penalty on performance, or just increase the resolution by 3x rather than say 2x.
Find the dosbox configuration file .conf and edit it... It will contain something like:
[render]
# frameskip: How many frames DOSBox skips before drawing one.
# aspect: Do aspect correction, if your output method doesn't support scaling this can slow things down!.
# scaler: Scaler used to enlarge/enhance low resolution modes.
# If 'forced' is appended, then the scaler will be used even if the result might not be desired.
# Possible values: none, normal2x, normal3x, advmame2x, advmame3x, advinterp2x, advinterp3x, hq2x, hq3x, 2xsai, super2xsai, supereagle, tv2x, tv3x, rgb2x, rgb3x, scan2x, scan3x.
frameskip=0
aspect=false
scaler=normal2x
normal2x is a good selection,
normal3x usually makes it too big. if it's still too big, try
none.
Also the fullscreen option may be of interest to you...
[sdl]
# fullscreen: Start dosbox directly in fullscreen. (Press ALT-Enter to go back)
# fulldouble: Use double buffering in fullscreen. It can reduce screen flickering, but it can also result in a slow DOSBox.
<snip>
fullscreen=true
fulldouble=false
