That size UPS is simply unfit for any kind better gaming PC.
Successfull transfer to battery power also needs likely some reserve.
Active PFC draws some current almost continuously to keep current draw in phase with voltage and keeping bulk/primary capacitor fully charged.
So "transfer time" delay of UPSes lowers capacitor's charge state, likely resulting in momentary higher power draw to restore charge.
Though that would likely anyway trip the UPS and relays clicking during transfer from line operation to battery is easy to notice.
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