ZFW Controller reported aircraft flying substantially lower than filed speed. When questioned; flight crew replied they were operating at economy cruise.

Narrative

Aircraft X had leveled at FL220 for a couple of minutes; but his ground speed had stabilized at about 365 KTS. Aircraft X's filed TAS was 425. I asked the pilot of Aircraft X if he was operating at a reduced airspeed; as his speed we well below his filed speed. The pilot stated they were using economy cruise and doing Mach .60. After determining his correct TAS for flight plan purposes; I changed his flight plan to show M.60 TAS. This has happened before; and seems to be happening more often now. I recently had a [same company] slow unexpectedly and cause me to have to vector a following flight away due to a rapidly developing overtake. It is most common on short-haul routes. Pilots; especially in this era of fuel and cost economy pushes; need to remember their responsibilities to forward changes in their TAS to ATC so their flight plans can be changed to reflect this. This is even more important as more advanced probing is used on flight plans. There is a standard for at what point pilots as supposed to advise ATC of changes; and this was clearly exceeded.