Air Carrier Captain reported during flight both engines having issues getting to commanded EPR resulting in reduced aircraft performance. They proceeded to their destination airport.

Narrative

Very cold morning; no visible precipitation. On takeoff roll; once PG called autothrottles; engine appeared to accelerate normally. On check thrust call; everything initially appeared normal. However; Left Engine was slower to go to commanded EPR (Engine Pressure Radio). PM pushed both throttles up to commanded EPR. Right Eng; slow as well; got to commander EPR. Left Engine was lagging. At this point the aircraft hit 100 kts. PM called 100 kts. Rest of take off normal. Left Engine EPR was 0.03 below commanded EPR. Flight continued. Over ZZZ; aircraft hit; mountain wave. PM used thrust to slow (not speed brake). When commanded EPR was rest to accelerate; Right ENGINE could not got past 1.55 EPR even through 1.60 EPR was commanded (FL400). This condition lasted 25-30 mins. Dispatch and Maintenance Control was consulted. ELB (Electronic Log Book) entry made. Eventually aircraft descended to FL300 (ATC directed due to approaching). TAI (Thermal Anti Ice) was turned on. Shortly afterward Right Engine EPR was reached with no issues. Landing was uneventful.