There is always going to be competition for mobs in populated zones.
If you're going to try to 'outperform' than what your job can claim - you are now putting yourself on the 'competitor' list.
Thus, you'd be making yourself the target for other 'competitors', and vice versa.

A tip to 'hold claim' over a yellow mob is based on emnity and total damage dealt to the mob in question based on my observations.

You can hold only mob at a time per PC character, anything outside of that will shift back to yellow eventually.
If you took 5% of the yellow mob, and someone comes and does, for example, 15% within a very short amount of time (like a ws), then the claim will shift due to the lead of emnity and damage dealt.

With that, if you bring the yellow mob to 20%, and someone starts auto-attacking, they will not be able to claim the mob nearly as quickly.

Though jobs with a strong ws ready, or a BLU , can easily one shot and take the kill.

Compete, or play it safe with your 100% claim over one mob.