Use of routing-mark is not compatible with fasttracking of transit (forwarded) packets in firewall which is present in the default firewall rules. So disable the rule in chain=forward of /ip firewall filter which says action=fasttrack-connection and the loading of pages should become equally fast via both ISPs.
The point is that not all packets eligible for fasttracking are actually fasttracked (i.e. actually bypass most of the firewall processing), so those which are not fasttracked do make it to the destination via their proper WAN because they get a proper routing-mark. The fasttracked ones take the default route and as they have a wrong source address, they are dropped on the way or at least ignored in the destination.
Depending on your hardware model and WAN link speeds, your CPU may be bored or glow red without fasttracking. If it glows red (/tool profile will tell you), read this.