Limited support
This feature is only present in Portal 2: Community Edition and Portal: Revolution.

sv_props_funnel_into_portals_deceleration

🟩 Executable on Server

sv_props_funnel_into_portals_deceleration <value>

When a funneling prop is leaving a portal, decelerate any velocity that is in opposition to funneling by this amount per second

Warning:

This is a cheat command and can only be used when sv_cheats is set to 1.