Model Linksys WRT1900ACS v2
Architecture ARMv7 Processor rev 1 (v7l)
Firmware Version SNAPSHOT r13658-5234593462 / LuCI Master git-20.171.46309-c351bee
Kernel Version 5.4.48
Internet PPPoE IPv4/IPv6 dual-stack 1Gbps
You see something strange in the top, is this value of 99% normal, what it represents, this value?
It's idle CPU time, not weird if the CPU is not working on something.
Most likely dd-wrt is using proprietary closed source modules for the chipset of the router, which OpenWrt is not using.
But it is not clear from all these screenshots which is better. There is a speedtest with dd-wrt ar 178Mbps.
dd-wrt might be using hardware offloading (The stock firmware most definitely is). Due to its closed source nature, Openwrt does not have hardware flow offloading (With an exception for the mt7621 platform). This means that performance on Openwrt in most cases is lower than on stock firmware. In exchange though, you got many more features and security updates that the stock firmware can't even dream of having.