USB 2 “speed”, i.e. bandwidth is not a bottleneck in your setup - unless you are sending an insane amount of stereo channels to your audio interface. USB 2 has a bandwidth of 480 Mbps, while a stereo signal at 48 kHz requires just 2.3 Mbps (ignore all the protocol overhead etc.). So if you send one or two stereo pairs to your interface, your USB 2 connection is mostly bored…
The more expensive interfaces typically have other differentiating features: better AD/DA converters, lower noise, integrated DSP capabilities etc, but communication speed is not the issue.
One thing that can contribute to better performance of an audio interface is the quality of the driver (the piece of software on your PC that handles the task of moving audio data to the interface) in combination with the specific hardware - there are some vendors that have a reputation of very efficient drivers, e.g. RME. That’s where investing in a “better” interface may pay off in lower latencies.
But the key of real-time audio processing is still the “computing and throughput capability” of your PC, i.e. its ability to make a lot of calculations and have data flowing through storage → memory → CPU → memory → output as quickly as possible.
The key challenge in real-time audio is managing the flow of data to your interface. The smaller the “chunks” of data are (buffer size), the more hectic the process of filling that buffer and handing it over to the audio interface becomes, and it puts more strain on the throughput of your system (not only CPU, but also the communication between the processes in your system - some processes need to wait for the output of others, which causes a delay). So this process of moving data to your interface becomes a bottleneck. Increasing the buffer size lets your system concentrate more on actually calculating the data (running plugins) and moving the data between plugins vs. just shoveling it out to the interface. The price you pay is more latency.
Essentially, no single solution will bring you a quantum leap in performance - a well-balanced, modern system with good drivers, using some of the typical real-time-audio tweaks can perform really well.
See the update that @cpaolo posted recently: Feedback computer config - #77 by cpaolo . My little “Live Cube” (ASRock Deskmini, first in the list) using a middle-of-the-road Core i5 processor with 6 cores compares pretty favorably - and it’s not a “high-performance monster” with 10 or more cores.
Cheers,
Torsten