We observed rapid variability (≲60 s) at the footpoints of transient, hot (∼8─10 MK) coronal loops in active region cores, with the Interface Region Imaging Spectrograph (IRIS). The high spatial (∼0"33) and temporal (≲5─10 s) resolution of IRIS is often crucial for the detection of this variability. We show how, in combination with 1D RADYN loop modeling, these IRIS spectral observations of the transition region (TR) and chromosphere provide powerful diagnostics of the properties of coronal heating and energy transport (thermal conduction or nonthermal electrons, NTEs). Our simulations of nanoflare-heated loops indicate that emission in the Mg ii triplet can be used as a sensitive diagnostic for nonthermal particles. In our events, we observe a large variety of IRIS spectral properties (intensity, Doppler shifts, broadening, chromospheric/TR line ratios, Mg ii triplet emission) even for different footpoints of the same coronal events. In several events, we find spectroscopic evidence for NTEs (e.g., TR blueshifts and Mg ii triplet emission), suggesting that particle acceleration can occur even for very small magnetic reconnection events, which are generally below the detection threshold of hard X-ray instruments that provide direct detection of emission of nonthermal particles.