Hey guys I'm new here and I've been weight training now for 3 and a half months, I'm starting to see results which is giving me the motivation that I need to continue but I have one question.

The person I weight train with tells me to go slow when lifting weights but when I slow down it seems to tire me out more than work the actual muscle so I'm wondering if lifting weights should be done slow or fast?

Thanks

It tires you out because of the physiological response... there is more friction on the negative phase of the exercise, so the slower you go, the more heat/friction/energy usage.

But the answer to your question is BOTH.

You should train slow at certain points during your cycle, and with explosive speed during other parts. They stimulate different muscle fibers and generate different training effects. There is no set rule to working out, other than don't set yourself a rule.

Technically doing the exercise slowly will increase muscle size as the muscle is put under a high amount of stress. If you do the exercise fast, the muscle endurance would increase and you would burn fat and not put on muscle as effectively...

That's somewhat of a misconception... powerlifters often train with high-speed/explosive reps, and they aren't lacking in the mass department.

It's all dependent on how much weight you're talking about and how you perform the rep. Explosive training recruits the fast-twitch white fiber muscles, which are the strongest and largest of the muscle cells.