I'm using a low end laptop to play minecraft so I need a texture pack that help me reduce lag and increase fps. Can you guys suggest me which one should I use?(legal of course)

You'll have much better luck improving your performance with mods. All legal ones can be found listed (and linked for your convenience) here: https://redlime.github.io/MCSRMods/ Hope this helps!

Yeah I've used the mods but I'm unsure if the texture pack can help reduce lag as well?

Special resource packs, especially ones which simplify block models, can reduce lag. However, any changes that could have this effect are not allowed for Minecraft speedrunning.

As of the new resource pack rules on 2 April 2023, the only allowed in-game changes that are relevant for 1.16 are:

• The text of the end poem.
• The ender dragon death sound.
• The dragon and wither boss bar names.
• The colour scheme of the inventory background, provided that it isn't transparent, and changed only to another solid colour or a gradient (i.e., no images).

None of these will have any effect on lag.