Where the microphone is, and how far away you are
Start here, because it is the thing you can change today.
A phone is held near your face. A tablet is propped on a stand, laid flat on a table, or held in two hands at reading distance, which puts the microphone further from your mouth. Speech recognition works from the sound that reaches the microphone, and on a stand your voice arrives mixed in with the room around it rather than on its own. Same software, different input.
Three things to do about it, in the order to try them.
Pick the tablet up when you dictate. It slightly defeats the purpose of a stand, and it changes the recording more than any setting will.
Find out where the microphone actually is. Placement varies enormously between tablet models, and it is frequently along an edge you are covering with a hand or resting against the flap of a case. Once you know, you can stop covering it.
Lean in. Speak the way you would to be heard across a table rather than the way you would speak to somebody sitting next to you, and aim at the tablet rather than past it.
If the tablet is staying on the stand regardless, pick the largest model size, since that is the one built for accents and background sound.
Wi-Fi only changes the calculation
A great many tablets are Wi-Fi only, and a Wi-Fi tablet gets used in exactly the places where there is no Wi-Fi: on a train, in a car, in a garden, at somebody else's house, on holiday. Those are also the hours when a tablet is being used for reading and writing rather than for work.
Cloud dictation is simply unavailable in all of them. Not slow, unavailable.
LocalType runs the recognition on the tablet itself, from a model in storage no other app can open, so a tablet with no connection behaves exactly like one sitting at home on the kitchen counter. Internet access is requested for exactly one purpose, collecting the model you chose, which you do once over Wi-Fi and never think about again.
For a Wi-Fi-only device that is not a nice extra. It is the difference between dictation existing and not existing.
A big flat keyboard is harder to type on, not easier
A large on-screen keyboard sounds easier than a small one and usually is not. The keys are too far apart for thumbs and too flat for touch typing, so most people end up hunting and pecking with two fingers at a device that is otherwise comfortable to use. Dictation is worth more on a tablet than on a phone for that reason, not less.
Once the distance is handled, four kinds of writing go noticeably better here than on a phone.
Longer messages and email. A tablet is where people answer the mail they postponed on a phone, and dictating three paragraphs into a big screen means you can see the whole thing while you check it.
Notes and drafting. A tablet on a table is a decent writing surface, and alternating between dictating a paragraph and editing it with a stylus or a finger is a genuinely good way to work.
Forms. Anything with many fields is faster spoken than typed across a large flat surface, particularly addresses and reference numbers.
Reading and replying. The classic tablet evening, where the reading was always pleasant and the replying was always the annoying part.
Shared tablets and the personal dictionary
If several people use the same tablet, anything the keyboard learns is shared. A personal dictionary containing your colleagues' names is visible to whoever picks the tablet up next, and words added by one person change the suggestions everybody else sees.
That is not a privacy failure. It is what a shared device is, and better decided deliberately than discovered. On a family tablet, keep the added vocabulary to things everybody uses: the street, the town, the school, the family names. Anything work-related belongs on your own phone.
The same logic explains one small behaviour you will notice. In a password field no microphone key is offered, which matters more on a device other people in the house also pick up than it does on a phone that never leaves your pocket.
Ageing hardware, and which model to pick for it
Tablets get replaced far less often than phones, so the device you are setting this up on was frequently mid-range when it was new and has been in the house a long time since.
That matters because memory is the constraint for on-device recognition, and a tablet usually has less of it than a phone bought in the same year. The practical answer is the smallest model, which runs comfortably where the largest would stall.
The two arguments pull against each other, and on a tablet both are real. Distance argues for a large model; old hardware argues for a small one. If both apply to you, the fix is not the model at all. Pick the tablet up when you dictate and run the small one.
An old tablet also frequently has very little free storage, and installing a model briefly needs room for two copies of it. Clearing space first saves a confusing failure halfway through the download.
Setting it up
Same two steps as a phone, since a tablet is an Android device like any other.
Enable the keyboard in the language and input settings, then select it from the keyboard selector while a text field is open. Download the speech model over Wi-Fi before you need it, and grant microphone permission the first time you press the key.
Choose the size with the distance in mind. On a phone the middle option suits most people; on a tablet used at arm's length the largest is often worth the wait. For scale, the 190 MB middle setting converted 6.9 seconds of speech in 4.1 seconds on a 4 GB test device, and the largest takes roughly double the time you spent speaking. Only one of them lives on the device at a time.
Nothing else about the product behaves differently on a larger screen. No account is needed, advertising is absent, no behavioural tracking runs inside the product, and no archive of your recordings is kept. Android's cloud backup is deliberately denied the app's data, which on a tablet means the same fresh start after a factory reset that it means on a phone. Eighteen languages come from one multilingual model, and which of them you are speaking is worked out from the recording, so there is no menu and no setting to change between messages.
LocalType runs on Android 8.0 and newer and works on tablets as long as there is a microphone in the device, which covers almost all of them.
Two cases where it is not worth it
If the tablet never leaves a desk and has a proper keyboard attached to it, typing is fine and dictation is solving a problem you do not have. Physical keys beat both of the on-screen options, and that is not in dispute.
And if it is a shared family device used mainly for video, installing a keyboard that changes typing for everybody in the house is a larger intervention than the benefit justifies. Set it up on your own phone instead, where the dictionary is yours and nobody else has to adapt to a decision you made.