Speed is $...

"Not very fast" is pretty subjective. Do you have a reference? 0-60, 1/8 or 1/4 times, etc etc?

One thing I recommend is one of the performance meters like G-tech which can give you acceleration informantion

After that the basics, see above

Run a compression and leak down test.

Then tuneup. Timing. Play with A/F ratio. You have a display which gives you that?

What are tire diameter? Consider lower rear gear

Auto? Consider a stall converter.