Eval bug: suboptimal DFlash multi-slot performance

### Name and Version

v0.3.0

### Operating systems

Windows

### GGML backends

CUDA

### Hardware

RTX 3090

### Models

_No response_

### Problem description & steps to reproduce

Low tok/s and GPU utilization (70-80%), meaning significant bottlenecks are in place

### First Bad Commit

_No response_

### Relevant log output

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<summary>Logs</summary>
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