Movement in the game is done through buttons, and double tap holding the button to go in allows Pac-Man to continuously run in said direction.

I couldn't find this documented somewhere here, or through searching through forums/wikis.

There is a way to increase Pac-Man's top speed, and it is through mashing the movement buttons as fast as you can. Through my testing, the faster you mash, the faster Pac-Man will travel.

Here is the framecounts I got for round 1 when testing this theory. (Note: these were recorded in 30fps) I started timing on the first frame you can see the timer at the bottom of the screen, and ended time on the first frame Pac-Man transitions into his walk animation at the end of the round.

Double tapping and holding: 24.267

Rhythmic Tapping/Slow Mashing (Roughly 5 bp/s): 23.433

Mashing (Roughly 11-13 bp/s): 22.3

Using a Turbo input (30 bp/s TAS level): 21.9

Obviously Turbo isn't a legal speedrunning tactic, but I thought it'd be a good idea for a somewhat consistent benchmark to compare to since everyone mashes differently at different speeds.

This was all tested on the Midway American release of the game.

TL;DR: Mash the direction you are going in as fast as possible, whenever possible throughout the entire duration of your run.