There can be many reasons for slowness.
If the recorder has a lot of cameras recording continuously, this can push the hard disk speed limits. The system might also be unable to record fast enough. Disabling the Separate time lookup database setting can also slow searches as the lookup table is rebuilt in the database. Enabling this setting for all recorders may speed up browsing recordings.
The cause may also be too much recording. If you have set up a rule that triggers continuous recording, you can try disabling it. Once the system is not recording everything, check if the playback works better.
It is also possible that the hard drives are old/broken and read speeds have slowed down.