iMac Fusion Drive Partitioning Issue

In last month I bought a iMac (Retina 4K, 21.5-inch, 2017) with 1 TB Fusion drive. (3.4 GHz Intel Core i5, 8 GB 2400 MHz DDR4, Radeon Pro 560 4 GB)

After the initial setup, I tried to to partition the HDD to 4 partitions using Disk Utility inside the OS. (400,300,200 and 127GB) At that time total drive size shown as 1027GB.

At that time partitioning failed mentioning some reasons (like can not continue, try again etc) then total drive size was reduced and finally I had to go for Internet Recovery after trying many options and ended up now showing two separate drives as shown below.

And that SSD partition can be accessed in OS too. Now I have Installed OS Mac OS High Sierra 10.13.

Later I understood this 28GB comes as a SSD in 1 TB Fusion drive. As shown now I have installed Mac OS on disk1s1 and SSD shown as a separate drive as disk0.

I'm worried that this has slow down the iMac thinking whether Mac OS supposed to be on SSD part of the drive.

How can I bring this two drives together and get the initial factory condition to have one drive?

Thanks.