Anandtech referring to single-core performance between core types:

"In the aggregate scores, an E-core is roughly 54-64% of a P-core, however this percentage can go as high as 65-73%. Given the die size differences between the two microarchitectures, and the fact that in multi-threaded scenarios the P-cores would normally have to clock down anyway because of power limits, it’s pretty evident how Intel’s setup with efficiency and density cores allows for much higher performance within a given die size and power envelope."

Anandtech referring to the multi-core performance:

"Focusing on the DDR5 set, the 8 E-cores are able to provide around 52-55% of the performance of 8 P-cores without SMT, and 47-51% of the P-cores with SMT. At first glance this could be argued that the 8P+8E setup can be somewhat similar to a 12P setup in MT performance, however the combined performance of both clusters only raises the MT scores by respectively 25% in the integer suite, and 5% in the FP suite, as we are hitting near package power limits.,, and there’s diminishing returns... given the shared L3 [cache]"

Extrapolating maybe too much from this article, it sounds like the E Core can hit around 3/4ths of a P Core around a similar frequency, but the P Core's higher frequency makes the gap larger. On the other hand, the E core probably can't clock higher because it's designed for efficiency, so all that extra power dedicated to the P Cores is being put to use by boosting the frequency.

It's also very interesting seeing how the main bottlenecks in multi-core performance with the big.LITTLE core format are the power constraints and shared CPU cache. The article also mentions how the increased bandwidth of DDR5 actually makes a noticeable difference in multi-core performance when the shared cache is oversaturated.

Source: https://www.anandtech.com/show/17047/the-intel-12th-gen-core-i912900k-review-hybrid-performance-brings-hybrid-complexity