When updating stock and currency prices under Investments / Equities, KMyMoney prints the source text of the corresponding web page (providing the online quotes) to the standard error stream (prefixed "kmymoney_webpricequote: quotedata"), which in my case gets redirected to the systemd journal. For me, this creates about one megabyte of log data for every price update. The expected behavior is not to waste resources with huge unsolicited debug messages.
Start KMyMoney with e.g. QT_LOGGING_RULES="kmymoney_webpricequote=false" kmymoney or configure the logging using one of the other provided methods. See https://doc.qt.io/qt-5/qloggingcategory.html for details.