B757 First Officer reported a rejected takeoff due to a 'RT BLEED OFF' EICAS message along with a lag in takeoff power being achieved. Crew returned to gate and changed aircraft after discussion with maintenance.

Narrative

On Day 0 around XA:00 after about an hour taxi due to congestion and poor weather we were cleared to line up and wait runway XX in ZZZ. All checklist had been completed prior to taking the runway and everything appeared normal. The weather conditions were calling for LLWS Advisories; so we had briefed and planned on performing a max thrust takeoff to mitigate any potential hazards associated with the LLWS. We were cleared for takeoff and Captain stood the thrust levers to about 1.10 Engine Pressure Ratio (EPR). We noted a delay/lag on the right engine compared to left. Once both engine EPR matched 1.10 Captain called for 'auto throttles'. When selecting the auto throttle button both thrust levers increased at the same time. As it increased Captain called for 'check thrust'. Our max EPR was planned to be for 1.77 EPR around 1.60 EPR I noticed the right EPR on the engine instruments lagging behind the left engine and manually began increasing the right thrust lever to assist it in reaching 1.77 EPR while noticing a large throttle split between the left and right throttle. The left engine had already been set to 1.77. As I was assisting the right thrust lever to match around 1.70-1.73 EPR we received a 'ENG BLEED OFF - R' EICAS message around 85-90 knots. Captain; with the knowledge of the large throttle split; plus the right engine not being able to achieve takeoff thrust and having a caution message; decided to reject the takeoff. The rejected takeoff was uneventful and followed SOP; we came to a stop on the runway and coordinated with ZZZ tower to exit the runway. Once off the runway we ran the 'rejected takeoff' QRC and out of abundance of caution elected to have Airport Rescue and Firefighting (ARFF) inspect our brakes before returning to the gate. Once at the gate we were met with maintenance where we discussed what happened; we decided it was best to have an aircraft swap before continuing on.