Oops, I remembered some other examples.
From existing examples in the MM, with heavy armour,
20ft becomes 15ft
30ft becomes 20ft
40ft becomes 30ft
50ft becomes 35ft
60ft becomes 40ft
90ft becomes 60ft
It seems that my earlier estimate of 3/4 speed is erroneous. Another possibility could be 2/3 base speed, round up to nearest 5ft. But regardless, you should be able to infer the reduced speed you want from the values above.