I like the trick gemini twins, there's a version I do where you can do it with four cards. I believe it's Alan Ackermans' but I'm not sure. Anyways the point is, there is some dealing involved, which gives me a slow point in my otherwise fast paced act. While the cards are being dealt there is obviously some time where nothing is being said(unless you're counting). Should I insert some funny lines? Do the lines have to be funny? Is silence okay? Hope this makes sense.

I'm convinced that a "slow point" to usw your own words is never a gold thing - unless you use it to build some tension before a climax. Form your description I'd say that is not the case in your routine. A joke for the sake of a joke is not it either - unless the rest of your routine is humorous, too. So: At that point in your routine, where do you want your spectators attention? Why? What do they need to remember? What do they need to forget? Is a reiteration in order? Or a real or a pseudo-explanation for the dealing-process? Or a quip about how you hate card-tricks that require a lot of dealing ... Hope this gives you an idea or two.

You could wistle the Jepardy tune....it kills time... it gets a laugh
Is you style fitting of something like this.

I have people think it really funny to deal right down to the very last card.

One little gem I heard was:

I wonder when NOW would be a good time to stop dealing.

If innuendo suits your style and suits the occasion you can refer to them 'going all the way' and we only just met.

Just make everyone count out loud -- loudly! -- with you. No silence, and audience participation.

Nothing says Entertainment more than counting out loud.

Just chat naturally with people during this time, don't force it.

Are you humorous in other parts of the routine or performance?
Do you have a character traits that you act out or bring in to your routines?
If not, do you want to begin doing so?

Who is doing the dealing?

Assistant- "You count very well, do you work at a bank?" Are you drug dealer? Accountant? School teacher? Etc. depending on your audience.

You- I am getting good at this counting, I should work at a bank. ...Be a drug dealer, accountant, school teacher, etc....

*Break from counting and ask, such as, "What comes after five?" They then say "Six".
You say- Okay, I just wanted to see if we are paying attention.
...No, DRINKS come after five!

This is a pretty funny idea. Need to learn how to whistle now