Anything is possible. Since you did use security over the years, many things can change when it comes to security and modes it operates on and client HW and drivers they need to interact with. So having it OFF over the years, you wouldn't probably notice anything since the Wifi doesn't have to process anything, now you've turn on security and now the WiFi on the router and the Client HW have to process this thus performance can be slowed. AES has the best performance of them all however other things can effect performance, external sources and interferences, drivers and 3rd party security programs on PCs and Laptops.
I would update drivers first to ensure they are good to go. Then to a factory reset of the 615 and set it up from scratch. Use the suggestions mentioned. If your not using A or B modes, use Mixed G and N modes only or if all of your devices support N, use single mode N. Auto 20/40Mhz and WPA2/AES only.
Speed only on a wired connection, turn everything OFF accept 1 wired LAN cable PC and speed test. It's not recommended to speed test on WiFi, many factors can contribute to inaccurate results. As long as the wired connection tests fine, you'll be good to go.
There is newer FW as well...I recommend v3.11 to start with. If you do choose to update, please follow this:
FW Update Process