Spliting tables with *almost* same values
Certainly not one table for each document.  Perhaps one for each document type.

In general you are correct - reading or writing to larger tables typically takes longer than the same operation on smaller tables.  There are techniques to help with this - indexing for example.  Modern DBMSs can easily handle tables of several million rows, however, so unless you are pushing these limits it is better for your design to match your requirements than to work around the limitations of the technology.

If you commonly update several different documents of different types simultaneously then a single table would be beneficial.  If each document type is processed independently then separate tables would be better.