Hello Pedro
3TB Data with a filesize under 300kb -> you have to backup over 10 millions of files. Protection a lot of small files is always slower than backup fewer bigger files. This is for example because of the filesystem (for example NTFS is not very fast with small files), i/o from disk subsystem (raid level, how many disks and so one).
Do you have one 3TB Volume or more smaller once? In generall more volumes leads to more data streams in backup which gives you better performance.
if your server is a vm, best choice would be to backup those data over vStorage API on Image Level. Then you are also faster if you need to do a disaster restore from the whole vm. but keep in mind that it is a bit more "complicated" to do single file restores. There is a new feature with DP 9 for Single Item Restore from vStorage API backups
regards