Slower download speed than upload (TP-Link TD-W8970)

Hello, I installed the last OpenWRT release on a TP-Link TD-W8970 router.
I set a bridge between this and another router, but I get always a slower download speed than upload.
There are two devices involved: a Samsung S2 tablet and an Honor 9 lite smartphone.
I had the same issue with an older version of Openwrt (21.02.4) too.
What can I do to improve the download speed?