The output of the left hand op-amp will typically have a peak-to-peak swing of 13.5V on a 15V supply. If you look at the slew rate, that equates to roughly 1 µs taken for each swing. If the wave being produced measures as 300 kHz, bearing in mind a single cycle will comprise two swings, the switching time would account for 60% of the cycle. The output probably looks more like a triangle wave with sine-wave-like tips, due to the integration of the falling ramp as the left op-amp switches. Only the remaining 40% of the wave is actually linear ramp.
Although wasteful in terms of layout size, the ideal solution is to use a fast-switching comparator for the left-hand op-amp such as an LM319, though you'll always come up against a frequency limit at some point.