If you routinely store frequently used files on external drives, look to upgrade any USB 2.0 devices to USB 3 or Thunderbolt, if your Mac
supports those interfaces. While a hard disk won’t draw the full benefit of the bandwidth they offer, performance is usually a lot better than from drives that use USB 2.0. Reduce the number of USB devices that are sharing the same internal controller (in System Information, select USB on the left and check which devices are attached to each ‘hub’). Disable the option in System Preferences’ Energy Saver pane to put hard disks to sleep when possible, so they’re always ready for action; some drives override this in firmware, so also check any management software provided by the manufacturer.