Is this true even if the openVPN is setup in bridged mode?
I can't speak to Bridge mode as I've never tried it, but theoretically then your clients would be on the same IP network so I would say no it probably wouldn't be true for that scenario. I don't think pfSense has bridge mode as an option out of the box but you might be able to configure that using the custom options field.
Yes, it is true that my Internet connection is only 100 Mbit. However, the network between LAN and DMZ is at Gbit speed today. I don't want LAN/DMZ communications to be affected negatively.
Well….. pfSense runs on a PC, can you use Gbit NICs? Or are you running it on an embedded device?
-Rich