Some of the most common factors that could influence speed are:
1) Firebird database needs to be backed up & restored (effectively fixes indexing or data issues)
2) The server is not optimised as a database server (i.e. the disk configuration is designed for a file server - copying large files, rather than high I/O traffic)
3) Anti-virus software is slowing down performance
4) The server is running other processes (such as exchange server etc) which also require a high level of disk access.
5) Refer to the following Knowledgebase Article "Installing Firebird 2.5.8 in a multi user environment"
6) Review the Current Database Sweep Interval setting. refer this article for more information