The first technique is prevention of distraction. I remember at the age of 15, I got my first computer. I was so fascinating for it, I spent so much time on my computer until my stomach growls, my eye hurts, and my finger numbs.
This is a part of the speech I will be given. Is it grammatical to use present tense here: "my stomach growls, my eye hurts, and my finger numbs", so that it sounds more vivid?
It is grammatical to use present tense - the form is called the Historical Present. But you need to be consistent and write the entire sentence in present tense - you have the past tense verb forms 'was' and 'spent'. Change them to present tense 'am' and 'spend'.
There are other mistakes in your speech but I am focusing on tense.
You are talking about the habit or scene that happened every time. The choice is the modal verb 'would' there.
Said that...
...I spent so much time on my computer until my stomach would growl, my eyes hurt, and my finger numb.
The change of tense in a narration written in past tense to a passage in present tense is a literary device with the effect of hightening the depiction of a scene. By changing to present tense you reduce the distance just as in films when the camera changes the distance and brings a scene in close-ups.
Your example is a bit cumbersome. You can't begin a sentence with past tense and continue in present tense. The transition from past to present tense is often done in an artful manner. And then a longer dramatic scene is given in present tense, not only a sentence or clause.
