RT-AC5300 - TLS slow

[gheppy]

Hi,
My firmware is 384.8_2 and I have this problem.
I have dual WAN active with two different ISPs (as in photos).
When I activate an ISP everything is good with TLS and web pages, it loads quickly.
But when I have both ISPs active, I have a slow TLS and slow navigation and can take up to 50 seconds to load a net page that loads in 2-3 seconds normally.
Are there any settings to fix this problem?
Thank you

 

[L&LD]

What are each ISP's speeds that you're paying for? Are they equal?

How many concurrent users are you testing with?

 

[gheppy]

The first ISP is 1000 Mb Up and 450 Mb Down
The second ISP is 1000 Mb UP and 500 Mb Down

There are 4 desktop computers and 2 Smart TV in the house, and the problem is present on all devices.
I tested with one computer online and the problem is the same.

I do not have this problem with the firmware RT-AC5300_380.70_0