bad engine performance today

Today I was doing some touch and go's in a C152 with one of my
students to prepare for his first solo. It was a very hot day (35C),
but density altitude was only 2500 or so.

I noticed as we were climbing upwind, the plane was climbing very
slowly, even right at Vy. The runway is very long (11,800 ft), but
when we reached the opposite threshold, we were only about 500 AGL.
Normally at the end of the threshold we are between 900 and 1000 AGL,
and thats with switching to cruise climb after 500 AGL...

Then when we got to downwind, my student pulled the power back to 2300
RPM (cruise) and we were only able to hold about about 70 knots (90
knots is normal).

Just before this all happened, I was up with another student in a
different plane (still a 152 but a different tail number), and the
performance was about the same as it normally is.

The thing that confuses me about all this, was how the engine RPM was
perfectly normal. Full throttle was about 2500 which is where it
normally is. Wouldn't an engine that is producing less power have less
RPM's as long as it's a constant speed prop?

I know it isn't a airspeed indicator problem because I could clearly
notice a difference in ground speed from comparing the two flights.
After the flight I went around the place to see if there was anything
on the airframe that could have caused a lot of drag, but it all was
normal. What could it have been?