To actually know how fast is your Synology is to run a benchmark tool to assessed the CPU performance. I’m going to run the Geekbench 4 through a pre-made docker.
Once you installed Docker on your Synology, click on ‘Registry’ and search for bhcopeland/docker-geekbench4 I’ve found it by searching geekbench4
Compare to my RS815+ with a Intel Atom C2538
Geekbench Intel Atom C2538
The Intel Celeron J3355 is faster than the Intel Atom C2538 in the single-core performance, due to only having dual cores it slightly slower overall. The Intel Atom C2538 has quad core and has a TDP of 15W it’s takes a 50% more power consumption level. The newer Celeron does well with just having a TDP of 10W, reduce power consumption lower heat output for near enough the same performance. Though there is a massive speed boast with AES for the DS218+, oddly both CPU has Intel® AES New Instructions