The scenario strongly suggests the device has been compromised by malware delivered through a social- engineering message (an "unexpected overdue invoice" attachment). After the attempt, the phone shows system-wide performance issues affecting the browser and other apps, and a restart does not fix it-this matches common mobile malware symptoms such as sluggish response time and abnormal behavior that persists across reboots. When symptoms persist beyond a soft reset, the most reliable remediation is to return the device to a known-good state by performing a factory reset (wipe). Quentin Docter explains that a factory reset is much more effective and will remove malware along with applications, and that when a security symptom indicates malware, the device should be factory reset and then protected with antimalware before reinstalling apps.The All-in-One guide similarly teaches an escalation path: stop/uninstall suspect apps, and if these steps don't resolve the behavior, use a factory reset to cleanse the device.#turn27file4 L9-L14# Mike Meyers' Lab Manual also lists factory resets among the key tools for resolving persistent mobile app security issues.
References: Quentin Docter - CompTIA A+ Complete Study Guide (mobile malware symptoms; factory reset removes malware)#turn27file2 L9-L15#; Travis Everett & Andrew Hutz - CompTIA A+ All-in-One Exam Guide (factory reset to cleanse device when symptoms persist)#turn27file4 L9-L14#; Mark Soper - Mike Meyers' A+ Lab Manual (app security troubleshooting tools include factory resets)