Air carrier First Officer reported encountering mountain wave activity in the vicinity of ALS VOR that resulted in large altitude and airspeed deviations.

Narrative

While at cruise altitude approximately 30 NM east of ALS (Alamosa VOR) we encountered mountain wave activity. Aircraft Y in front of us encountered the wave activity at the same location which gave us a few minute's warning of what was to come. We were monitoring the aircraft's performance and noted +/-10 kts of airspeed fluctuation. All of a sudden the airspeed dropped 30+ kts and [we] began to descend. The Captain disengaged the autopilot and lowered the nose to try and arrest the decreasing airspeed. I immediately notified ATC of the wave activity and our descent. We bottomed out 700' below our cruise altitude. Once the airspeed began to increase we climbed back up to our assigned altitude. During the entire event we kept ATC informed of what was happening.Cause: By monitoring the ATC frequency we knew that there was wave activity ahead of us. This event reinforced the importance of sharing ride reports to ATC so other crews on the same route or in the general vicinity can be alerted to any adverse weather or ride conditions. One thing we could possibly have done was request a route to the south which may have avoided this wave activity all together.