hi Nate, many thanks for replying.
my arrangement is extremely simple & minimal. i have no home network per se at all, merely one FttP (fibre to the premises) modem-router, connected by wire to my FttP connection to my house. my primary pc, the one in question here for all my kdelinux posts & threads, is ethernet-connected to this modem-router. my ISP contracted plan is 500/40 mbps, which it routinely exceeds eg, 540/47, in my browser per Ookla. this pc has archlinux & kdelinux on it. when booted into Arch, & doing routine system updates sudo pacman -Syu, i always hit my full ISP d/l rate. otoh, since my comparatively recent return to kdelinux on this second SSD [i have been jumping between it, kinoite, & kalpa], as per several of my threads my system update UX has been simply miserable… persistently ghastly-slow d/l speeds <300 kbps, & thus most updates have failed with timeout errors.
however, i have tentative delightful news now to report… success! 
i am now in the latest build, 202606010254
Summary of my contemporaneous notes
today i had a new idea; i suspect it might be an IPv6 problem, in that kdelinux seems to want to use that protocol for these updates [ Do these network transcripts look ok, or problematic? ], but afaik my ISP does not support it! so, after research, i cobbled together this string, which first shows the IPv6 status [active], then disables it for this runtime, then does the updates:
ip -6 addr show && sudo sysctl net.ipv6.conf.all.disable_ipv6=1 && sudo sysctl net.ipv6.conf.default.disable_ipv6=1 && flatpak update && updatectl update
initial appearances were poor, in that the flatpak updates remained hideously slow, then finally timed out with one still to do. i manually then did that one ok, then manually ran just the nuclear system update command run0 /usr/lib/systemd/systemd-sysupdate update (ISPv6 still remains disabled, thus forcing the process to use my good IPv4).
this time the difference was substantial - finally, & for the first time, the system update achieved my proper ISP d/l speeds of 500+ mbps! the transcript indicates success, but will it boot ok…?
YES, it boots! i am now in the latest build, 202606010254, yaaaaaaaay!
i have now created this alias:
alias upd="ip -6 addr show && sudo sysctl net.ipv6.conf.all.disable_ipv6=1 && sudo sysctl net.ipv
6.conf.default.disable_ipv6=1 && run0 /usr/lib/systemd/systemd-sysupdate update && flatpak update"
i am very pleased, but not yet willing to state i have found a solid workaround, until & unless the next few updates also prove successful.