How to read a large number of xlsx files without error on EXCEL server?

I have about 15,000 xlsx files to extract information from. XLSREAD was used to read data. It worked fine and fast at the beginning, but became slow after several thousand files have been read. At the same time, there was a warning saying that EXCEL server failed and 'Basic Mode' was used. Data were still able to be read in 'Basic Mode', but it was very very slow. Does anybody know what may cause the failure in EXCEL server after reading a large number of files? Thanks!

PS: The Matlab version is R2013a. The EXCEL version is 2010. The OS is WinXP SP3.