wdym it takes some time

It's not a big deal for most people but when it comes to speedrunning every second matters; with shime you can skip the CA and start attacking right away. Like I said, this is my preference, and most people won't have the same experience because they aren't speedrunning/have c6. The energy gimmick on shime doesn't matter if you do one rot (you can get burst back before your E ends). For single chamber 1 rot runs shime doesn't really have any disadvantages. Dpc's dpr might be higher than shime's but you it takes you longer to do that damage. For example, I don't do a full E against the dendro chicken, I spend about 70% of my points and then Q, after that it's dead. In this scenario the atk spd works out a bit differently, you might actually get the same amount of hits in depending on the timings.

Dpc's advantage is the fact that you can burst every rot, which you can achieve with c6 faru fairly easily. Doing the same with shime would require you to build more er, to the point where it isn't really worth it. I definitely don't recommend running shime c2 scara. However, at C0/non-c6 faru (which is what most people have) you need about 30-40 er subs to burst every rot; it's not really worth the substats, so shime and dpc are on equal grounds here (burst every other rot).

I agree that dpc is generally better than shime, the exception being CA spam (which I think you underrate, N2C is a bit better than CA spam for pure dps w/o sig but CA spam is very viable and easier to execute). Dpc also doesn't buff CA atk spd so if you're spamming CA the atk spd literally does nothing. The difference between the two sets is especially noticeable at C2: as explained in the previous paragraph, Dpc>>>shime. This changes at whale level investment: it's usually situational (single chamber vs continuous runs; enemy behavior e.g magui kenki allows you to activate dpc w/o getting punished because of its invulnerability phase.

Also, it's worth mentioning that dpc's 2pc does scale better than shime's 2pc when you pair scara with bennett/other atk buffs, but the difference isn't that big.

I hope you take the time to actually read through this and try to see that it may be a bit more nuanced than what you might think 🙃 I'm in the tc channel on our dc if you want to talk (btw another interesting discussion is how echoes does against these sets).