1. Close every program but DOSBox

2. Turn off all the scalers:

Edit dosbox.conf and set:

[sdl]
fulldouble=false
fullfixed=false
fullwidth=0
fullheight=0
output=surface

[render]
aspect=false
scaler=none

3. Run DOSBox in a fullscreen mode:

hit Alt+Enter after running DOSBox

(if you have black stripes at the top and the bottom of the screen go to your display driver settings and change refresh rate for these two resolutions: 320x200 and 640x400. ("60Hz" and "default" is bad so set something else: "70Hz" or "75Hz" or higher). If you have a LCD (thin) monitor, connect a CRT (normal/big) monitor for a while, change refresh settings, make sure they work correctly, and connect your LCD monitor back.)

4. Set mpu401/GeneralMIDI instead of Sound Blaster for music in your game's setup if it is possible.

5. Set GUS/Gravis instead of Sound Blaster for sound/digital audio in your game's setup if it is possible.

5. If you really want to use DOSBox in a windowed mode (which is slower than fullscreen) you should set your desktop to 16 bit color.

6. If you want you can reduce audio quality:

Edit dosbox.conf and set:

rate=11025
sbrate=11025
adlibrate=11025
etc.

7. You can use unstable cynamic core:

Edit dosbox.conf and set:

core=dynamic

It may speed up the game but it may also make the game not runable or to crush often. So if your game crushes/doesn't work dont use it. Even if your game seems to work fine save often 😉

8. Setting cycles/frameskip:

You should go here now:
The Newbie's Pictorial Guide To DOSBox
-> STEP 3 - Performance

Could this be added to "DOSBox Guides" forum?