I have just done the speedtest from the link you posted and it gives 91,09 Mbps. Is that good?
Very good, it's about global average speed according to https://www.speedtest.net/global-index#fixed. I wish mine as fast as yours
For the CPU and RAM :
Intel Celeron CPU J1900
1.99 GHz
RAM : 4,00 GB (3.90 GB available).
Intel Celeron CPU J1900
1.99 GHz
RAM : 4,00 GB (3.90 GB available).
With information you mentioned, very likely Intel Celeron J1900 & 500GB HDD are the cause of the slow sync. Open Task Manager and check whether your CPU/Disk usage is near 100% all theme when running Bitcoin Core.
At a speed of 91.09 Mb/s it should only take 54 minutes to download 300GB of blocks.
A Celeron has only one thread and even though bitcoin core's block verification is single-threaded it is guaranteed to be constantly at 100% CPU until it ends.
Celerons also have the unfortunate property of having a lower clock speed than almost every other processor, and that makes the verification much slower.
You can verify whether the CPU is the bottleneck by opening Task Manager and go to the Performance tab, then check the CPU graph and see if it's always full, and during that time the Network graph should be empty, or minimal.
Your disk should not be the problem unless the disk usage graph is full.
If however your RAM graph is almost full that means you are out of memory and Windows is moving around bitcoin core's dbcache around swap space. This is an extremely slow operation which slows down the program and ultimately the whole computer more than a bad disk or cpu.