We'll need more information to really help.
What does "slow" mean, exactly?
What speed did you want from it?
Under what exact conditions do you want that speed? (scooter/board and rider weight along with anything else being carried, road/path surface type, wind/weather conditions, slope (hills? how steep and long?, other terrain, traffic/not, etc).
Are there any other symptoms of a problem?
Were there any previous problems?
Is anything getting hot?
Is there a lot of voltage sag on the battery under load? (if it has a bar-type battery meter display (or voltmeter), do the bars (or volts) stay the same regardless of what you're doing, or do they decrease by any amount (how much?) when using it vs when not vs when using it hard, etc?)
A link to the exact motor, battery, and controller you purchased would also be useful. If not, then pictures of any labels and markings on each item would be useful, as well as of each overall piece, and any documentation that was supplied with them, if any was.
What is the actual thing the motor/etc is being used on? Scooter? Skateboard? Etc? If something else like a bike, I can move this to the appropriate forum instead of the Stand-up E-scooters and E-skateboards subforum.
If I were to make a bunch of assumptions about what you have, and how it's used, and how it might be installed, the best guess out of all the common problems would be that the phase/hall combination is wrong between motor and controller, so it's a false-positive, and so can't provide the power it could if it were wired in the right combination. This results in a lot more motor (and often controller and wiring) heating, from the wasted power, so it's usually possible to tell there is some kind of problem, even when you don't know what it is.
The next most common problem is a battery that simply can't handle the load placed on it, so that it sags in voltage so much under load that the total power output isn't enough to reach the speeds it could if it didnt' sag. This usually also results in shutdowns as the battery gets closer to empty than full, so is also usually possible to tell there's some kind of problem at that point.
The next most common problem is a limiter wire or setting, that either limits power or speed or both. Sometimes it's a built-in software thing where if you aren't pedalling and/or don't have the PAS sensor installed/setup, the power the system will make and/or the speed it will go from just throttle is limited severely.