Just a very long shot idea: could it be connected to Sentinel: ipset turris-sn-dynfw-block_v4 is full ? Recently, the dynamic FW ipsets grew an order of magnitude. This could overload older hardware like T1.1 .
So I just announce that I have solved the problem with slow internet connection as follows: I replaced Turris 1.1 with Asus TUF AX3000 after nine years. Now everything works like clockwork. Moreover, the control is user-friendly, everything is in Czech. Simply satisfaction. Thanks for the Turris team for nine years of support, but we simply have to part ways.