hello @Eli_Roz welcome to the forum 
Please use the search - lot of issues have been discussed already. e.g. you will find reports Xeon based system might be not enough: Nextcloud Overload
I can’t judge but maybe you didn’t install HPB and your clients flood the system with PROPFIND requests which is definitely not good with slow CPU.
I don’t have good recommendations, just the feeling a Celeron(R) J4125 CPU in NAS is not good enough for 25 users and 3 Mio files.