Atom CPUs are core-for-core comparable to the A72 cores in the RPi4. ICP isn't really comparable since the instruction set is different, but performance is similar. The RPi has 4 of them, only the most modern Atoms have that.
I'm not seeing USB 3 on that particular Optiplex, either. Gigabit ethernet, yes. No HDMI. DVI, VGA. Maybe displayport? Hard to tell. Do have PS/2 keyboard and mouse ports, which is kinda nice.
No USB3. I don't think it's any good as a thin client any more, it's only good as a low power server. Before the Pi4 it was much more attractive, especially because of GigE and because the Atom chip in it is pretty good compared to the ARM in the Pi3. But after reading the replies here I'm thinking it's not as good a choice as it once was.
