DOSBox build to be independent from the your system speed (with obvious upper restriction of it).
It measures speed in "Cycles". And in my experience, most early 90s games are happy with 10 000 - 15 000 of it.
But for best automation DOSBox have other two modes. One is "Auto" (where it tries to guess the best speed), and second is "Max" (that set cycles as much to your physical limit as possible).
Current mode must be shown in the head of DOSBox window anyway. And most probably it's max.
To correct this, find file dosbox.conf and check what value "cycles=" parameter set. Change it to reasonable numbers.
Then you can be interested in two other parameters nearby. "cycleup=" and "cycledown=". It defines how far will be increases and decreases of speed with hot keys. I use 500 for up and 100 for down usually, but you are free to use any numbers.
And at last - hotkeys itself.
Ctrl+F11 to decrease cycles,
Ctrl+F12 to increase them. No, they will not work if cycles are auto or max.