Reborn's Anti-Wallhack feature prevents cheaters from using wallhack cheats. It's implemented fully server-side, which means that cheaters can't circumvent it the same way as traditional client-side anticheat protections.
However, it's not perfect and it adds significant computation load to server. It also causes player flickering (player model blinking, showing/hiding) when players go behind a cover (such as walls).
You can try different anti-wallhack modes by setting sv_antiwh cvar on your server:
set sv_antiwh <0-6>
In modes 1, 2 and 3, there is additional cvar sv_antiwhskipping that is used to define a player's ping above which anti-wallhack skips checks for particular player.
By default it's set to 400.
set sv_antiwhskipping <0-999>
In practice it means that players with ping higher than defined by this cvar will be excluded from anti-wallhack system. Players with high ping suffer from heavy flickering, and this cvar can help those players, but it may mean that some cheaters will get excluded from anti-wallhack protection.