This shouldn't be a problem. Excellink will partition the amount of data it retrieves from the spreadsheet and send them in chunks to the PLC. We tested with 50 DMs being transferred from the spreadsheet to the PLC and have no problem.
Is your PC linked to the PLC via serial port or Ethernet?
If it is by serial port, check if your PLC program is writing out data via the same serial port that could be interfering with TLServer communication with the PLC.
You can also test by transferring a blank program to the PLC and check if the ExcelLink communication works much better.