I think it's mentioned clearly in the article.

> RISC-V builders have four or eight cores with 8, 16 or 32 GB of RAM (depending on a board)

> The UltraRISC UR-DP1000 SoC, present on the Milk-V Titan motherboard should improve situation a bit (and can have 64 GB ram).

RISC-V SOCs just typically don't support much ram. With the exception of the SG2042 which can take 128GB, but it's expensive, buggy and now old.

So I am sure it's a combination of low ram and low clockspeeds.