The fastest tire for Brompton i.e. ETRTO 349 should be the Schwalbe One introduced on the CHPT3 2019 and now available as spare parts.

There is also now a folding Marathon Racer tan wall introduced on the Explore 2020 and also now available as spare parts.

Besides Schwalbe, there is the new Continental Contact Urban which is s good trade-off performances/puncture protection.

Last very fast tire in ETRTO349, the Greenspeed Scorcher 120. Its 40mm wide so theoretically too wide for the Brompton but I saw pictures of Brompton equipped with this tire.