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Audio Interfaces DO wildly vary in low latency performance.
Only two places in the world test interfaces for this database, Vin at Dawbench HQ in Australia and Me at Scan in Bolton.
We've got identical test bench computers to measure the performance with.

Essentially, at low (under about 12ms) latencies (so you don't get a noticeable delay when playing stuff in on the keyboard) good interface choice can actually mean you can double or triple the ammount of plugins and instruments you can run.

ok, before i post the performance chart, you need to understand how it works.
Each interface has been tested to see how many RXC compressor plugins (RXC Column) and how many notes of virtual instrument polyphony (CV / NCV columns - With and without Convolution Reverb enabled) it can run (in Kontakt) at different buffer settings and what the latency times are for each buffer size.

Each interface is then given it's own overall LLP (Low Latency Performance) Score , which is calculated compared to the best interface we have ever tested (RME HDSPe AES card and ADi-8 converter), which scores a ten.
The best performing interfaces are at the top of the list

I can't paste the full detailed breakdown as its bigger than 1000 pixels and the forum wont let me, but you can follow THIS LINK and check out the numbers for yourselves.

*note, i've recently re-tested the Native Instruments Komplete Audio 6, but the list hasnt been updated yet. Its now scoring about 6.7 on it's latest drivers!

So, lets compare two interfaces, in this case the RME AIO and the Focusrite Saffire 6

We can see that the AIO can run 155 RXC compressors before the audio breaks up and 300 notes of Kontakt Polyphony in the CV column (With Convolution Reverb) and 340 notes NCV (Without Convolution Reverb) at a combined round trip latency (RTL) of 8.6ms.
In comparison the Saffire 6 can only manage 68 RXC compressors, zero notes of kontakt polyphony with the convolution reverb and only 80 notes without it at a slightly higher latency of 9.8ms.